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Field Museum of Natural History
Publication 232
Geological Series Vol. IV, No. 4
ON THE HEAD OF THE MACROPETALICHTHYIDS
with certain remarks on the head of
the other arthrodires
BY
Erik A: son Stensio
Royal State Museum, Stockholm
Oliver Cummings Farrington
Curator, Department of Geology
Chicago, U. S. A.
October, 1925
PREFACE
The Museum specimen (P 1154) on which the following paper by
Dr. Stensio is chiefly based, is from the Onondaga (Corniferous)
Middle Devonian limestone of Leroy, Genesee County, New York.
The original collector is not known to the Museum. The specimen
together with another (P 11 55), from the same locality, was obtained
by the Museum from Ward's Natural Science Establishment, Roches-
ter, New York, in 1894. Its remarkably complete preservation has
enabled Dr. Stensio to make the profound anatomical study that is
presented in the following pages.
The matrix of this specimen when freshly broken is seen to be a
dark brown, partially crystallized limestone. This limestone is some-
what argillaceous and bituminous, and slightly magnesian. Here and
there it exhibits the inclusions of chert which are characteristic of
Corniferous limestones. On weathered surfaces the matrix is chalk-
white in color, Upon which the dull black parts of the fossil fish stand
out in contrast.
In specimen P 11 55, only the dermal bones of the cranial roof of a
Macropetalichthys are preserved. This head is larger than that of
P 1 1 54, being 19 centimeters in length as compared with 12 centimeters
in P 1 1 54. The width (12 centimeters) is about the same for both.
The plates preserved in the larger specimen are of a dark, calcareous
substance, about 1 mm. thick. Except for a black pigment, this sub-
stance dissolves readily in dilute hydrochloric acid. On removal of the
remains of these plates, imprint of a pustulose external surface can be
plainly seen on the overlying rock. In the limestone matrix of both
specimens, numerous aggregates of more or less comminuted brach-
iopod shells and crinoid stems are visible. Among the brachiopods so
preserved, individuals of the species Leptaena rhomb oidalis and A try pa
reticularis can be identified.
Oliver C. Farrington.
87
ON THE HEAD OF THE MACROPETALICHTHYIDS
WITH CERTAIN REMARKS ON THE HEAD OF THE OTHER ARTHRODIRES
BY
Erik A: son StensiS
CONTENTS
Introduction 91
Description of the Macropetalichthyids 92
Macropetalichthys rapheidolabis 92
Primordial neurocranium :
General remarks 92
Occipital region 95
Labyrinth region 103
Labyrinth cavity 116
Orbitotemporal region 119
Ethmoidal region 129
Cavum cerebrale cranii and brain 134
Dermal bones of the primordial neurocranium 140
Dermal bones of the cheek 143
Visceral skeleton 144
Dentition 146
Sensory canals of the head 146
Macropetalichthys agassizi? 150
Macropetalichthys pelmensis 152
Epipetalichthys wildungensis, gen. nov., sp. nov.
Primordial neurocranium:
General remarks 152
Microscopic structure of the bone 153
Occipital region 154
Labyrinth and orbitotemporal region , 155
Ethmoidal region 156
Cavum cerebrale 157
Dermal bones of the cranial roof 157
Sensory canals of the head 158
Some general remarks on the Macropetalichthyids 160
Certain remarks concerning the other non-Macropetalichthyid
Arthrodires 164
The Phlyctenaspids 165
The Coccosteids 1 70
The Homosteids 180
The Mylostomids 180
The Ptyctodontids 181
The Jagorinids 182
Some general remarks on the non-Macropetalichthyid Arthrodires. 186
Concluding remarks on the affinities of the Arthrodires 187
Bibliography 191
Explanation of Plates 198
89
INTRODUCTION
As the holder of a scholarship from the University of Upsala and
with support from the Palaeontological Museum of Kristiania, I under-
took in 1922 a journey to the United States of America for the study
of certain groups of fossil vertebrates, especially Ostracoderms and
Arthrodires.
In examining the collection of the Arthrodires in Field Museum
of Natural History at Chicago I found a specimen of Macropetcdichthys
raphcidolabis, (Field Museum Cat. No. P 1154), that if prepared in a
proper way was likely to show the primordial neurocranium rather com-
pletely preserved. Through the courteous assistance of Mr. H. W.
Nichols, Associate Curator of Geology of the Museum, permission was
secured for me to carry out the preparation of the specimen which I
found necessary for my studies. While carrying on this work it soon
became evident that the specimen was of much greater importance from
the anatomical point of view than I at first thought; for most of the
canals for nerves and vessels, the entire cavum cerebrale and a part of
the labyrinth cavity were in such a good state of preservation that they
could be examined in detail. I spent ten days working with the speci-
men and during this time the photographs reproduced in Pis. XIX-
XXVI were made by the photographer of the Museum. (Cf. Annual
Report of the Director, Field Museum of Nat. Hist., Publication 213.
Report series, Vol. VI, No. 2, p. 121.)
The present paper is chiefly based on the specimen of Macropeta-
UcJithys rapheidolabis, in Field Museum of Natural History at Chicago,
that has just been referred to. But as this specimen was imperfect
regarding the occipital region, this was studied on certain specimens in
the American Museum of Natural History in New York, a few of
which — those figured in this paper — were sent to me in 1923 in Sweden
for a re-examination in certain respects.
The specimen of the new, interesting form Epipetalichthys wildun-
gcnsis described in the present paper was, in the most courteous way,
placed at my disposal by Professor O. Jaekel of Greifswald, and for
the opportunity to examine the specimen described below under the
name of Macropetcdichthys agassisi? I am indebted to Mr. S. Junker-
mann of Bielefeld.
9i
92 Field Museum of Natural History — Geology, Vol. IV.
It is a pleasant duty for me to express here my most sincere thanks
to Mr. H. W. Nichols, of Field Museum of Natural History, for all
the help I received from him and for the kindness he showed to me
during my stay at Chicago. Further I wish to express hearty thanks
to Dr. W. K. Gregory and Dr. W. D. Matthew for their help in
facilitating my studies at the American Museum of Natural History
in New York, and for their great kindness in sending material to me
in Sweden. Finally, I am also highly indebted to Professor O. Jaekel
of Greifswald, both for the material which I received from him for
this paper and for important information concerning the Wildungen
Arthrodires upon which he is working.
All the drawings for this paper were made by Mr. G. Liljevall,
of Stockholm.
DESCRIPTION OF THE MACROPETALICHTHYIDS
MACROPETALICHTHYS RAPHEIDOLABIS
Norwood & Owen
(Pis. XIX-XXVII, PI. XXVIII, figs. 3-5J PI. XXX, fig. 2; PI. XXXI, fig. 1)
PRIMORDIAL NEUROCRANIUM
General Remarks. The primordial neurocranium is strikingly
broad and low, its maximum width almost equalling the length and its
maximum height constituting only about one-seventh of the length and
between one-fifth and one-sixth of the maximum width (Figs. 1, 3,
5, 8; Pis. XIX-XXII; PI. XXIV, figs. 1, 3). The maximum height
is situated in the posterior and the maximum width in the anterior half
of the labyrinth region.
As is seen from text figs. 4, 5, 8, 9, and PI. XXIV, figs. 1, 3, the
primordial neurocranium is so arched both in a transversal and longi-
tudinal direction, that its dorsal surface is convex and its ventral sur-
face correspondingly concave.
Concerning the shape of the different divisions of the primordial
neurocranium, it may be especially pointed out in this connection that
the occipital and ethmoidal regions have a considerable length, while
the labyrinth and orbitotemporal regions are short. The latter region
is even strikingly short and at the same time very broad, its length
being only about one-half of the width, a condition which is due partly
to the slight development of the orbits and partly to the considerable
width of the skull as a whole. With regard to the orbits, it is in addi-
tion noteworthy that they have shifted some distance backwards and
medially and that they are much directed upwards.
Macropetalichtiiyids and other Arthrodires — Stensio 93
The primordial neurocranium consisted of cartilage and bone, the
latter containing fairly numerous cell spaces. The bone occurs, how-
ever, only as thin layers, which covered the cartilage on the cerebral
surface, within the labyrinth cavity, to a large extent on the external
surface, and in addition also formed a lining membrane" for all the
canals for the vessels and nerves piercing the walls of the neurocra-
nium. Accordingly, there may be distinguished an internal bone-layer,
a labyrinth bone-layer, an external bone-layer and canal bone-layers.
More closely defined, these different layers of bone, which evidently
are perichondral, have the following extension and relations:
i. The internal layer forms a continuous covering of the cerebra/
surface of the primordial neurocranium. In other words, it lines ihe
whole cavum cerebrale cranii.
2. The labyrinth layer forms a continuous lining membrane of the
whole labyrinth cavity (cavities for the semicircular canals included).
3. The external layer covers the outside of the primordial neuro-
cranium ventrally, laterally and posteriorly, but dorsally, on the con-
trary, only in the posterior narrow division of the occipital region and
probably in a short anterior part of the ethmoidal region. The remain-
ing parts of the dorsal side of the occipital and ethmoidal regions, as
well as the whole dorsal side of the labyrinth and orbitotemporal re-
gions, are destitute of a perichondral bone-layer, so that the cartilage
must have appeared there to the extent shown in Fig. 3. The por-
tion of the external layer covering the posterior side of the anterior
broad part of the occipital region, is connected with the inner layer by
a few, chiefly sagitally placed, laminae of bone, which passed through
the cartilage, and which naturally are enchondral.
4. The canal layers line all the canals traversing the cranial walls,
even canals and branches of canals with a very narrow diameter. In
those cases where the canals pass from the cavum cerebrale to the out-
side, the layers lining them are continuous with and connect the external
and internal bone layers with one another. In a similar way the internal
layer is continuous with the labyrinth layer by layers lining such canals
which run from the cavum cerebrale to the labyrinth cavity.
The different layers of bone just described actually form by their
connections with each other a single large bone extending throughout
the length of the primordial neurocranium. That this large bone was
formed by the coalescence of a few smaller ones seems not improbable,
since vestiges of ossification centres perhaps occur in certain places.
94 Field Museum of Natural History — Geology, Vol. IV.
elf-
s.pal - - -
c.pal3
N^S^
JU'-%*
c.
car.ext-M«
chy-
-f.arthnv
^cr.Sp
hcem.
Fig. i. Macropetalichthys rapheidolabis
Restoration of primordial neurocranium in ventral aspect. Occipital region
after specimens in the American Museum of Nat. Hist., New York, other regions
after a single specimen in Field Museum of Nat. Hist., Chicago. The dimensions
of the occipital region probably not fully correct in relation to those of the other
regions. Cartilage dotted. X^-
c.car.ext, canal for the arteria carotis externa; c.hy, canal for the vena
hyoidea; ch.w, ridge caused by the most anterior part of the notochord; c.pah,
opening for the r. palatinus facialis ; era, canal for the radix aortae ; cr.ol, crista
occipitalis lateralis ; cr.sp, cranio-spinal process ; dix, probable foramen for the
dorsal branch (lateralis branch) of the n. glossopharyngeus ; f.art.hm, probable
place of articulation for the dorsal end of the hyoid arch ; ham, haemal groove ;
ju, canal traversed in the anterior part by the vena mandibularis ; in the longer
posterior part by the v.jugularis (behind the confluence between this and the
vena mandibularis) ; jiix and juz, anterior and posterior openings of the canal ju,
the anterior one traversed by the vena mandibularis ; na, nasal opening ; nf, nasal
fontanelle; n.l, canal for the n.lineae lateralis; olf, olfactory capsule; pr, postero-
lateral process of the anterior broad division of the occipital region; s.pal, groove
for the r.palatinus facialis ; s.ra, groove for the radix aortae ; Sx, groove leading
some distance anteriorly from the external opening of the vagus canal; IX, canal
for the n. glossopharyngeus; Xxn, canal for the n.vagus and the vena cerebralis
posterior.
Macropetalichthyids and other Arthrodires — Stensio 95
The position of these presumed centres is seen in Figs. I and 3,
and will be further dealt with in the subsequent description of the dif-
ferent regions of the primordial neurocranium.
Occipital Region
The occipital region is very long, its length probably being continued
between two and three times into the length of the primordial neuro-
cranium (Figs. 1, 3, 5; PI. XXVII; PI. XXVIII, figs. 4, 5). With
regard to its shape otherwise it may be considered as composed of two
divisions, an anterior one and a posterior one.
The posterior one of these divisions (Figs. 1, 3, 5; PI. XXVII;
PI. XXVIII, figs. 3-5; PI. XXX, fig. 2) is much longer than the
anterior one but, on the other hand, considerably narrower and lower.
With the longer anterior part it does not by far extend up to the
cranial roof. At its posterior end it has a paired laterally and dorsally
projecting process (cr-sp), which is coalesced with its fellow of the
other side medially and so connected on the anterior side with a
descending lamina from the posterior bones in the dermal cranial roof
that it forms a support for these. The process much resembles the
cranio-spinal process in Acipenscr, Polyodon and Saurichthys (Huxley
1864, Fig. 82; Parker 1882, PI. 15, fig. 13; PI. 16, figs. 1, 3; Iwanzow
1887, PI. 1, fig. 2; Gegenbaur 1912, pp. 319-325; Stensio 1925) and
may therefore be termed a cranio-spinal process although it obviously
has evolved independently of that in the fishes enumerated.
The most postero-ventral part of the posterior division situated
beneath that from which the cranio-spinal process issues is not found
preserved in any of the specimens investigated, a fact which seems to
indicate that it lacked the external bone layer and thus was entirely
cartilaginous when seen from the outside (Figs. 1, 3, 5; PI. XXVII;
PI. XXVIII, figs. 3-5; PI. XXX, fig. 2). It probably had a paired
condyle or a paired fossa for articulation with the vertebral column,
as a cranio- vertebral joint might have been present as in ordinary
Arthrodires.
Two transverse sections through the posterior division are shown
in text fig. 2. As is understood from these and Fig. 5, the pos-
terior division is much flattened in a dorso-ventral direction, its height
being only about one-half of the width. As a consequence of the flat-
tening, the external surface has no lateral fields, but only a dorsal and
a ventral field separated from each other by a paired, well pronounced
lateral crista, the crista occipitalis lateralis (cr.ol). The dorsal one of
the two fields, which, as mentioned above, is situated deep below the
96 Field Museum of Natural History — Geology, Vol. IV.
dermal cranial roof except at the posterior end, is provided with a
longitudinal unpaired crista, the crista occipitales dorsalis (cr.od), while
the ventral field, on the contrary, has a wide and deep longitudinal
groove. This groove (ham, Figs, i, 2; PI. XXVIII, figs. 3, 4)
constitutes undoubtedly an anterior, cephalic portion of the haemal
groove of the vertebral column, and may therefore properly be termed
the haemal groove. Towards the transition of the anterior division of
the region, it becomes gradually shallower and seems to divide there
into two shallow branches (Fig. 1; PI. XXVII, fig. 4), a right and
a left one, each of which is continued forwards by the groove s.ra
which will be further dealt with below in another connection.
The part of the cavum cerebrale cranii situated within the pos-
terior division is long and low and narrow in comparison with those
in front of it. It is, however, somewhat narrower in its anterior, than
in its posterior portions, as is well shown by Fig. 10 and PI. XXVII,
fig. 1. A transverse section through its posterior half is seen in Fig. 2.
The notochord extended into the basal part of the neuro-cranium,
but was very slightly developed there (ch, Fig. 2) and reached
forwards only to about the transition between the anterior and posterior
divisions of the occipital region (PI. XXVIII, fig. 4). It was, thus,
in the neurocranium, limited to the posterior division of the occipital
region, while the parts of it that originally existed further forward
as far as to the dorsum sellae have become reduced. The persistent
part was surrounded by a thin bone layer of its own, and this bone
layer, which is well preserved in the fossil, forms a fine longitudinal
tube, a tube which is coalesced with the inner bone layer dorsally and
the external bone layer (for explanation of these bone layers see p.
93 ventrally at the bottom of the haemal groove (Fig. 2). In the
anterior part of the haemal groove the notochord has pressed the
external bone layer downwards as a ridge, the notochordal ridge (ch.w),
which is conspicuous when the haemal groove is seen from below or in
transverse section (Figs. 1, 2B ; PI. XXVIII, fig. 4).
The slight development of the cephalic portion of the notochord of
the fish, forms, it seems to me, a support for the view advanced above,
that there was a kind of articulation between the head and the body as
in typical Arthrodires. (Adams 1919, pp. 123-127; Dean 1901, Figs,
pp. 116-117; Eastman 1908a, pp. 113-149; 1908b, pp. 158-205; Jaekel
1902, p. 106; 1907, p. 171; 1919, pp. 96-108; etc.) Such an articula-
tion required of course a considerable weakening of .the notochord at
the transition between the vertebral column and the occipital region of
the neurocranium, and if such a weakening took place the cephalic por-
Macropetalichthyids and other Arthrodires — Stensi6 97
tion of the notochord must evidently have lost its importance and
decreased in width and extension, just as it has done in recent Chi-
maeroids, which, as we know, have a sort of cranio-vertebral articula-
tion.
From its very considerable length it would be expected that the
posterior division of the region consisted of a rather large number of
coalesced segments, and, accordingly, that it was pierced by numerous
canals for spino-occipital nerves. No canals of this kind could, how-
ever, be found on the material investigated, and it is therefore impos-
sible at present to decide anything with certainty concerning the num-
ber of segments composing the division.
The anterior division of the region, if we now proceed to this
(Figs, i, 3, 5; Pis. XIX-XXIV; PI. XXV, figs, i, 2; PI. XXVI, figs. I,
2; PI. XXVII, fig. i), is, as already pointed out, much shorter, but, on
the other hand, considerably higher and broader than the posterior one.
Anteriorly, its dimensions are equal to those in the adjacent part of the
labyrinth region, so that it is really in no way marked off from this.
Mi i f . \_i iti iTif > rr nr
heem.
Fig. 2. Macropetalichthys rapheidolabis
A. Transverse section through the anterior part of the posterior half of the
posterior division of the occipital region.
B. Transverse section through the anterior half of the same division (accord-
ingly taken posterior of A).
After specimens in the American Museum, New York. Bone layers with
black, continuous lines. Cartilage dotted. Xi-
ch, cavity for the chorda dorsalis; ch.w, ridge in the haemal groove caused by
the chorda dorsalis; cr.od, dorsal occipital crista; cr.ol, lateral occipital crista;
cv, cavum cerebrale cranii; ham, haemal groove.
The postero-latero-dorsal corner of each side is produced backwards
and laterally to a rather large process supporting the lateral parts of
the dermal cranial roof.
For the detailed description we may conveniently consider that the
anterior division of the region is composed of five walls, which are as
follows : a ventral, a posterior, a right and a left lateral and a dorsal.
Of these the ventral, the dorsal, and the two laterals are, as is evident
from the figures, directly continuous with, and in no way marked off
from, the correspondingly situated ones in the labyrinth region.
The ventral wall, which is the smallest one of the five, is as a
whole narrow in comparison with the dorsal, although it rapidly in-
98 Field Museum of Natural History — Geology, Vol. IV.
creases in width forwards (Fig. i; Pis. XXI-XXIII). As is well
shown in PI. XXIII; PI. XXV, figs, i, 2, and PI. XXVI, figs. 1, 2,
ic is strikingly thin, especially in the median parts, and, as already
pointed out, the notochord was entirely reduced in it. It is not pierced
by any canals and has its externa^ (ventral) surface, the shape of which
may be understood from text fig. 1, well bounded along each lateral
border by a sharp edge. (
The posterior wall (Fig. 3; PI. XXIV, fig. 3; PI. XXV, fig. 2;
PI. XXVII, fig. 1) is large and mostly rather thick. Its ventro-median
parts are coherent with the posterior division of the region, its dorso-
median parts are a little inclined forwards and its lateral parts stand
approximately vertical or may, most laterally, be inclined a little back-
wards. The wall is covered by the external bone layer on the outside
and the internal bone layer on the inside, as are also the other walls,
but, as mentioned above, it is, in addition, provided with a few sagittally
running bone laminae (Is, PI. XIX; PI. XXV, fig. 2), which connected
the internal and external bone layers with one another. These bone
laminae in the fresh specimens traversed the cartilage.
The external surface of the posterior wall has, as is seen from
PI. XXIV, fig. 3, a considerable extension, which faces a little upward
in the dorso-medial parts and straight backward or backward and a
little downwards in the lateral parts. It is concave both in the trans-
versal (Figs. 1, 3, 5; Pis. XIX-XXIV; PI. XXVII, fig. 1) and dorso-
ventral directions and by an anterior extension of the crista occi-
pitalis dorsalis (cr.od) dorsally, and the posterior, narrow division
of the region, vertically, it is divided into a right and a left half. The
internal surface of the same wall, that is lower and narrower than
the external one, faces forward and downward (Fig. 11; PI. XX;
PI. XXIV, fig. 3; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2) and, like
the external one, is slightly concave both in transversal and dorso-
ventral directions.
The posterior wall is perforated by a paired, rather wide, postero-
laterally running canal (n.l, Figs. 1, 10, 11; PI. XXIV, fig. 3; PI.
XXVII, fig. 1), which has its external opening rather far laterally
on the external surface of the wall. This canal is, as will be evident
from facts given below, a branch from the wide vagus canal (Xvn)
and transmitted the n. lineae lateralis.
The dorsal wall (Fig. 3; Pis. XIX, XX; PI. XXIV, figs. 1, 3;
Pi. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2) has a very considerable
breadth in relation to its length. In its median part it is rather thin,
but grows rapidly thicker towards the lateral parts. It is much arched
Macropetalichthyids and other Arthrodires — StensiG 99
in transversal direction so that the convexity faces upwards. As has
already been pointed out, it has no external bone layer (Fig. 3)
and no definite external boundary. Despite this, it is, however, fully
evident that its upper surface had no fossae or depressions for por-
tions of the trunk muscles penetrating from behind, beneath the dermal
bones, for the external bone layer of the posterior wall has no emar-
ginations in its upper border and reaches in all its breadth quite up to
the lower surface of the dermal bones above it. (PI. XXIV, fig. 3.)
The sensory canals of the cranial roof being, as we shall see, situated
in rather strongly developed ridges on the lower side of the dermal
bones, the upper surface of the dorsal wall of the primordial neuro-
cranium must have had grooves for these ridges in the way shown
in text figs. 3 and 5. These grooves in the subsequent description will
be referred to as sensory canal grooves. (In the Field Museum speci-
men (Pis. XIX, XX; PI. XXIV, fig. 1) the dermal bones of the cranial
roof with the exception of those parts situated deepest, namely, the
sensory canal ridges, have weathered away, so that we can from the
conditions there easily see that the sensory canal ridges must have been
situated in grooves on the upper side of the primordial neurocranium) .
The canal for the n. lineae lateralis (n./.), during its passage through
the posterior wall of the division, gives off several fine branches in a#
more or less straight dorsal direction. These fine branches (Id^-ldg,
Pis. XIX, XX; PI. XXIV, fig. 3; PI. XXVII), which number at least
six and are surrounded in their total length by a thin bone layer, pass
upward through the dorsal wall and open into an overlying portion
of the sensory canal system, thus having a course that fully proves
them to have transmitted lateralis fibres and that the canal from which
they are given off was traversed by a thick lateralis nerve. As the
latter canal is the only one that issues backward from the vagus canal,
and, in addition, as we shall see, forms the sole possible way backward
to the abdominal region for all the lateralis fibres that accompanied
the vagus roots proper at the exit from the cavum cerebrale cranii, it
undoubtedly transmitted the whole n. lineae lateralis. The portion of
the sensory canal system of the head innervated from it is thus the
cephalic division of the main lateral line (Cf. the description of the.
sensory canal system below). In the Field Museum specimen, in which
the dermal bones of the cranial roof have weathered away and certain
parts of the underlying dorsal and posterior walls of the primordial
neurocranium have split off, the canal for the n. lineae lateralis and its
dorsal branches are, as may be seen in the figures mentioned, beauti-
fully displayed.
ioo Field Museum of Natural History — Geology, Vol. IV.
Some distance medially of these dorsal branches, a rather fine,
paired canal (cl, Figs. 3, 5 ; Pis. XIX, XX) goes upward through
the dorsal wall and, after arriving close to the upper surface of this,
turns abruptly medially, continuing in this direction almost until it
meets its fellow of the opposite side in the median line. During this
superficial, medially directed, ^art of its course it sends out several
branches both forward and backward. It could not be traced quite to
its ventral end and hence a positive explanation of its function can
not be given. It seems most probable, however, that it transmitted
bundles of communis and general cutaneous fibres from the vagus to
the cranial roof, and thus transmitted a ramus supratemporalis vagi.
Besides this it may perhaps also have been traversd by vessels and, in
addition, perhaps by a branch of lateralis fibres, as it communicated with
the canal for the n. linese lateralis by a fine branch. The destination
and morphological importance of this possible bundle of lateralis fibres
will be discussed below in connection with the description of the sen-
sory canal system.
The lateral walls of the division (Figs. 1, 5; Pis. XXI-XXIII;
PI. XXIV, figs. 1, 2; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1-4; PI.
XXVII, fig. 1) are thick throughout their extension, the thickness
being, however, not equal in all parts but gradually increasing upwards.
They are covered by the external bone layer on the outside and the
internal bone layer on the inside. Each has its external surface directed
laterally and much downwards and its internal surface almost straight
medially. A distinct but rather narrow groove (s.ra) runs along the
ventral edge of the external surface, a groove, which, as we shall see,
continues forward on to the external surface of the lateral wall of the
labyrinth region and backwards to the haemal groove, at the anterior
end of which it meets its fellow of the opposite side in the median
line. This groove undoubtedly lodged the radix aortse (lateral dorsal
aorta) of its side, as will be evident from the account of the labyrinth
region given below.
At the transition to the labyrinth region, each lateral wall is pierced
by the vagus canal. This canal (Xvn, Figs. 5, 10, 11, 12, 13; Pis.
XX, XXI; PI. XXIV, fig. 2; PI. XXV, figs. 1, 2; PI. XXVI, figs.
1-4; PI. XXVII, fig. 1), which is wide and on account of the thickness
of the lateral wall at this place has a fairly considerable length, runs
almost straight laterally to the external surface of the lateral wall,
where it opens with a wide, rather forward facing foramen (XVD,
Figs. 1, 5; Pis. XXII, XXIII; PI. XXIV, figs. 1, 2). A proximal
short part of it is much higher than broad (PI. XXV, figs. 1, 2; PI.
Macropetalichthyids and other Arthrodires — Stensio ioi
XXVI, figs. I, 2) and exhibits no certain evidence of subdivision by
longitudinal septa.
On the other hand the remaining distal, and by far longer, part, has
a more circular section than the former and is subdivided into two
incompletely separated divisions, a narrow and a wide one, by two
longitudinal ridges, divisions which perhaps in the living animal were
totally separated from each other by a membrane of connective tissue ex-
tending between the ridges. In the proximal part of the distal half of
-cr.sp
Fig. 3. Macropetalichthys rapheidolabis
Primordial neurocranium in dorsal view. Occipital region after specimens in
the American Museum, New York, and a specimen in Field Museum, Chicago;
other regions after the latter specimen. Perichondral bone with a dark tone.
Cartilage dotted. XV2.
cl, canal probably for a dorsal branch of the vagus and a bundle of lateralis
fibres; cr.od, dorsal occipital crista; cr.ol, lateral occipital crista; cr.sp, cranio-
spinal process ; d.end, canal for the ductus endolymphaticus ; f.end, fossa endo-
lymphatica; olf, olfactory capsule; orb, orbit; par, pineal opening; pfg, cavum
precerebrale ; pr, postero-lateral process of the anterior broad division of the
occipital region.
io2 Field Museum of Natural History — Geology, Vol. IV.
the canal the narrow division (Xv, Fig. u; PI. XXIV, fig. 2) is
situated dorsally of the wide one (Xn, PI. XXIV, fig. 2), but during
the passage outwards they gradually change their mutual position so
that the former finally lies behind the latter and occupies the posterior
portion of the common external opening, which is distinctly broader
than high. (PI. XXIV, fig. rf{
From conditions in other fishes we may conclude with great prob-
ability that in M. rapheidolabis the narrow one of the two divisions just
described in the vagus canal transmitted the vena cerebralis posterior
(v. encephalica posterior) and the wide one the vagus proper and in
the proximal half the n. linear lateralis too. For the vena cerebralis
posterior in all fishes which have been closely investigated with regard
to this, arises by the confluence of anterior and posterior branches from
the dorsal side of the medulla oblongata and enters the cranial wall
dorsally of the vagus roots. It then within the cranial wall usually
turns a little forward and crosses the vagus roots on the dorsal side
so that when reaching the outer surface it is situated just in front of
these roots and not as in M. rapheidolabis posterior to them. In cer-
tain forms it runs through the same canal with the vagus roots, in
other forms it is more or less completely separated from them by a
septum of bone, cartilage or connective tissue. (Cf. Stensio 1922, p.
172; 1925, p. 21; Grosser 1907, Fig. 4; O'Donoghue 1914, p. 442;
Pollard 1892a, PI. XXIX, fig. 23; Allen 1905, pp. 87-89; Rex 1891,
Pis. 15, 16; etc.) Similar conditions were also found in a specimen of
Chimcera monstrosa, which was dissected by me.
During the passage through the lateral wall, the vagus canal in
M. rapheidolabis gives off from its proximal half two branches, one on
the posterior and the other on the anterior side. The former of these
O.Z, Figs. 10, 11; Pis. XXI, XXIII; PI. XXIV, fig. 3; PI. XXVII,
fig. 1), which issues rather low, is the canal for the n. lineae lateralis
and has already been dealt with above in connection with the descrip-
tion of the posterior and dorsal walls. As pointed out there, it runs
postero-laterally to the outer surface of the posterior wall, sending out
several fine branches upwards to the cephalic division of the lateral line.
The other branch (dx, Figs. 10, 11; PI. XXIV, fig. 2), which is
much narrower and issues much higher up and more proximally, runs
some distance almost straight dorsally, then not so very deep below the
dorsal surface of the dorsal wall, bifurcating into an antero-lateral (dx&,
PI. XXIV, fig. 2) and an antero-medial (dxh, PI. XXIV, fig. 2)
ramus, each of which has a superficial course beneath a sensory canal
division, to which fine ramuli are sent out in a dorsal direction (PI.
Macropetalichthyids and other Arthrodires — Stensio 103
XXIV, fig. 2). And, as it is fully evident that these ramuli must have
transmitted nerves to the sensory canal organs, the anterior branch
(dx) from the vagus canal is consequently proved to have transmitted
a dorsal branch from the n. lineae lateralis. The relations of this
dorsal lateralis branch will be treated also in the description of the
sensory canal system below.
Into the proximal half of the vagus canal and, more exactly, into
the dorsal part of this just as it leaves the cavum cerebrale, there opens
a wide canal {v. lb, Figs. 10, 11 ; PI. XX, XXIV, fig. 2; PI. XXV,
figs. 1, 2; PI. XXVI, figs. 1, 2, 3), which comes from the postero-
dorso-medial corner of the labyrinth cavity. The function of this canal
is not fully clear, but it seems very probable that it transmitted a vein
from the labyrinth to the vena cerebralis posterior, because in the
Lorcati — the only teleostomous fishes investigated in detail with regard
to the veins of the head — there is a small vein which goes from the
labyrinth in a similar way back to the v. cerebralis posterior (Allen
1905, PI. Ill, figs. 23a, 23, 25). This vein in the Loricati receives
blood, as it seems, merely from the ampulla posterior, while the pre-
sumed vein in question in M. rapheidolabis , to judge from the large
width of the canal v.lb, must have received blood from the larger part
or the whole of the membranous labyrinth.
At the transition between the anterior and posterior divisions of
the region, the external bone layer on both the ventral and dorsal side
seems to show a certain very faint radiation from a median unpaired
centre. Hence, if this observation is true, there would be two unpaired,
vestigial centres of ossification in the region, the dorsal one of which
would correspond most closely to the centre of a supraoccipital bone,
the ventral one to the centre of a basioccipital bone. Better preserved
material than that investigated by the author is, however, needed for
a positive decision in this case.
The part of the cavum cerebrale cranii enclosed in the anterior
division of the region increases rapidly both in height and breadth
forwards. Its shape is well shown by Figs. 10, 11, 12, 13, PI. XX,
PI. XXIII, PI. XXIV, fig. 3; PI. XXV, figs. 1, 2 and PI. XXVI,
figs. 1, 2.
Labyrinth Region
The labyrinth region is very short and broad, its length being con-
tained about two and a half times in the maximum breadth. It is also
rather low, as its maximum height amounts only to between a fourth or
a fifth of the maximum breadth. The maximum height is situated far
back, almost at the transition to the occipital region, the maximum
104 Field Museum of Natural History — Geology, Vol. IV.
breadth, on the other hand, in the anterior half of the region (Figs. I,
3, 4, 5; Pis. XIX-XXI, XXIII; PI. XXIV, figs, i, 3; PI. XXV, figs.
1, 2; PI. XXVI, figs. 1, 2). As shown by the figures, the region is
arched both transversally and longitudinally, so that the dorsal side is
convex and the ventral side concave, the arching being, however,
stronger in the former than in the latter direction. The antero-lateral
corner of each side projects forwards as a large postorbital process,
forming not only a posterior but also to a large extent a lateral boundary
of the orbit, a disposal which is due to a postero-medial displacement of
the orbit. The postorbital process is ventrally, throughout its length,
continuous with the extensive orbital floor (Figs. 3, 8).
For the detailed description we may consider the region to be
composed of four walls: a ventral, a dorsal and a paired lateral.
The ventral wall, if we first turn to it (Figs. 1, 4, 5; Pis. XXI,
XXIII; PL XXV, figs. 1, 2; PI. XXVI, figs. 1, 2, 3), is comparatively
thin throughout almost its entire extension, but is thinnest in the
median parts. At the transition to the occipital region it is fairly
narrow, but grows rapidly broader forwards, finally attaining a very
considerable width. It is covered both by the internal and external
bone layers and perforated by certain canals in the lateral parts at
the transition to each lateral wall. These canals will, however, not be
described here, but in connection with the lateral walls, which are also
perforated by them.
Fig. 4. Macropetalichthys rapheidolabis
Transverse section through the posterior part of the labyrinth region. Dermal
bones marked with vertical lines. Layers of substitution bone with continuous black
lines. Cartilage dotted. XM-
Mi, L3, P2, S, bones of the dermal cranial roof (approximate extensions).
For their position cf. text fig. 15. c.pp, sensory canal commissure, probably cor-
responding to the posterior head line of pit organs in fishes in general; cv,
cavum cerebrale cranii; d.end, canal for the ductus endolymphaticus (ventral
part) ; d™, the antero-lateral branch of the canal d\ which transmitted a lateralis
branch to the above-lying sensory canal ; lab.cav, labyrinth cavity ; Ic, cephalic
division of the lateral line; s.ra, groove for the radix aortae (lateral dorsal
aortse) ; St, groove leading some distance forward from the external opening of
the vagus canal.
Macropetalichthyids and other Arthrodires — Stensio 105
The dorsal wall of the region (Figs. 3, 4, 5; Pis. XIX, XX,
XXIV; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1-3) is broad throughout
its extension, its breadth being, however, larger in the anterior than in
the posterior half. With the exception of its lateral parts, situated
above the labyrinth cavities, the wall must have been rather thick, and,
as pointed out already in the general description of the neurocranium,
it was covered only by the internal bone layer, so that the dermal bones
of the cranial roof rested on its cartilage. The external (dorsal) sur-
face must have had rather pronounced grooves for the sensory canal
ridges on the lower sides of the dermal bones of the cranial roof, as
indicated in text figs. 3, 4 and 5. Far back on the dorsal surface there
may, in addition, have been a paired pit (f.end) at the bottom of which
the canal for the ductus endolymphaticus (d.end, Fig. 3; PI. XX;
PI. XXVI, fig. 3) opened, for this canal does not seem to extend fully
up to the ventral side of the overlying dermal bones (Pis. XIX, XX).
In other words, there seems to have been a fossa endolymphatica.
Besides by the canal for the ductus endolymphaticus mentioned,
which will be described more in detail below in the account of the
labyrinth cavity, the dorsal wall must have been pierced by several
fine canals for vessels and nerve branches. Two of these, both for
lateralis branches to a portion of a sensory canal, are well seen in the
Field Museum specimen (in, Pis. XX, XXII).
The lateral wall of each side is very thick, especially in its dorsal
and anterior parts, and is covered by both the internal and external
bone layers (Figs. 1, 3, 4, 5; Pis. XX, XXI, XXIII, XXIV).
In its interior it contains the large labyrinth cavity (lab.cav, Fig. 4;
PI. XX; PI. XXVI, figs. 3, 4; PI. XXVII), which is completely sep-
arated from the cavum cerebrale cranii by a rather thick septum of
cartilage, covered by the internal bone layer on the cerebral surface
and the labyrinth bone layer on the labyrinth surface. The internal
surface of the wall is directed almost straight medially. The external
one consists of two fields, an anterior and a posterior one, the latter
of which faces ventrally to a marked degree, while the former, which
forms the posterior surface of the orbital cavity, faces mainly for-
wards. The two fields are separated from each other by a rounded
edge running from the antero-dorsal corner of the postorbital process
a short distance in a ventral direction. By another, although indistinct
dorso-ventral edge the posterior field is in its turn subdivided into an
anterior and a posterior part, the former of which is convex both in
dorso-ventral and antero-caudal directions and faces almost straight
downwards, while the latter, on the contrary, is concave in the same
106 Field Museum of Natural History — Geology, Vol. IV.
directions and faces much less downwards, but in addition a little
backwards (Figs, i, 4, 5; Pis. XXI, XXIII, XXIV). On the
latter part we find along the ventral border the anterior continuation
of the groove s.ra (Figs. 1, 4, 5; Pis. XXI-XXIV) already re-
ferred to above in the description of the lateral wall of the anterior
division of the occipital region, and at the transition to the anterior
part of the same field this groove deepens and is suddenly transformed
into a closed canal (era), which goes forward in the wall. We shall
below return to this canal (era) and the groove (s.ra) leading to it
and try to make out what their functions were. Before doing this we
must, however, examine the canals for cranial nerves that perforate
the lateral wall, beginning with the glossopharyngeus canal and pro-
ceeding forwards.
The glossopharyngeus canal (IX, Figs. 1, 5, 7, 10, 11; Pis. XXI-
XXIII; PI. XXIV, figs. 1, 2; PI. XXV, figs. 1, 2; PI. XXVI, figs.
1, 2, 4; PI. XXVII), which is fairly narrow, has its internal opening
situated rather far anterior of the vagus canal and much nearer to
the floor than to the roof of the cavum cerebrale cranii. It seems to
go almost straight laterally to the posterior lower part of the labyrinth
cavity, continuing from this cavity with about the same direction as
before to the external surface of the lateral wall, where it opens in the
anterior part of the groove s.ra, and accordingly close to the ventral
edge of the surface. The external opening is incompletely subdivided
into two divisions (Figs. 1, 5; PI. XXIV, fig. 1), an anterior and
a posterior one, the latter of which is slightly smaller than the former.
Somewhat dorsally of the external opening of the glossopharyngeus
canal there are traces of another, although finer, canal (dix, Fig. 1 ;
Pis. XXI, XXIII; PI. XXIV, figs. 1, 2; PI. XXVII), which seems to
run upwards to a portion of a sensory canal, piercing during its passage
the dorsal part of the lateral wall and the overlying lateral part of
the dorsal wall. Accordingly, this canal has a course which seems to
indicate that it may have contained a lateralis nerve. And as it is situ-
ated almost straight above the external opening of the glossopharyngeus
canal, we may conclude with a rather high degree of probability that
this lateralis nerve arose from fibres which had their passage through
the lateral wall, together with the n. glossopharyngeus, as is the case in
several other recent and fossil fishes, e.g. Selachii. (Ewart and Cole
1895, pp. 475-476; Norris and Hughe 1920, p. 358, Figs. 51, 52, 53),
Polypterus (Pollard 1892a, p. 397, PI. 28, fig. 13; Lehn 1918, pp.
395-396; Allis 1922a, pp. 283-284), Lepidosteus (Veit 1907, p. 187;
191 1, PI. D, figs. 2, 6, 10), Amia (Allis 1897, p. 684), Acipenser,
Macropetalichthyids and other Arthrodires — StensiG 107
Polyodon (Allis 1920, pp. 138, 142; observations made by the author:
cf. also Stensio 1924), Saurichthys (Stensio 1925) and certain
Teleostei (Herrick 1901, pp. 207-208, PI. XIV). In other fishes in
which the conditions are known, the corresponding lateralis fibres pass
through the vagus canal associated with the n. lineae lateralis.
The possible lateralis branch which has just been dealt with may
perhaps be thought to have caused the bipartition of the external open-
ing of the glossopharyngeus canal. And, if so, it seems probable that the
n. glossopharyngeus proper passed through the larger anterior and the
lateralis nerve through the smaller posterior division of the opening.
The canals for the n. acusticus (VIII) and the n. facialis (VII)
begin proximally in a common, distinct recess (afr, Figs. 10, IX J
PI. XXIII; PI. XXV, figs. 1, 2; PI. XXVI, fig. 2) on the ventral
part of the lateral side of the cavum cerebrale. This recess, which may
be properly termed the acustico-facialis recess, communicates in the
fossil freely with the cavUm cerebrale, but was perhaps in the fresh
specimens separated from this by a membrane belonging to the dura
mater. For comparison, it may here be mentioned that a recess of a
similar kind occurs in several recent fishes, but that in these it often
has a larger extension forward so that not only the acusticus and
facialis canals but also the trigeminus canal take their origins from it.
On account of this fact it is called in such forms the acustico-trigemino-
facialis recess, and it may further be noted that it sometimes is sep-
arated there from the cavum cerebrale by a thin lamina of connective
tissue or bone, a lamina formed by the dura mater. (Cf. Allis 1909a,
pp. 44, 4<>47> i24; i9!4a, pp. 232-236, 239, 240, 243, 246-248; 1914b;
1919a; 1922a, p. 228.)
The acusticus canal (VIII) of M. rapheidolabis, which is rather
wide, goes from the posterior part of the acustico-facialis recess almost
straight laterally to the labyrinth cavity, piercing the ventral part of
the septum that separates the labyrinth cavity and the cavum cerebrale
from one another. Its position in relation to the labyrinth cavity is
shown by PI. XXVI, fig. 3, in which the septum separating that cavity
from the cavum cerebrale, however, has been almost entirely removed.
The facialis canal (VII), which is of about the same width as the
acusticus canal, issues from the anterior part of the acustico facialis
recess (Figs. 10, 11 ; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2). On
account of the thickness of the lateral wall at this place it gets a con-
siderable length. It runs first for some distance forward and slightly
laterally close to the antero-ventral part of the labyrinth cavity
(Figs. 10, 11 ; PI. XXV, fig. 1 ; PI. XXVI, fig. 3), then at the antero-
108 Field Museum of Natural History — Geology, Vol. IV.
ventro-medial corner of this cavity it suddenly takes a more lateral and
ventral direction (Figs. 10, n; PI. XXV, figs, i, 3; PI. XXVI,
figs. 3, 4) and continues with this course to the external opening (text
fig. 1; Pis. XXI-XXII; PI. XXIV, fig. 1), close posterior to the
antero-ventral corner of the posterior field of the external (lateral)
surface of the lateral wall. The^more laterally and ventrally running
part is situated immediately anterior of the labyrinth cavity between
this and the orbit. As may be understood from PI. XXVI, fig. 3,
the part of the lateral wall separating the orbit and labyrinth cavity
from one another is rather thin.
Approximately at the place where it turns more laterally and down-
ward, the facialis canal gives, off from its antero-dorsal parts a rather
wide branch in an antero-dorsal direction. This branch (c.ophth.lat,
Figs. 8, 10, 11; PI. XXIII; PI. XXV, figs. 1-3) soon enters the
orbitotemporal region, ascending there after a short distance to the
dorsal wall and continuing forward into the ethmoidal region rather
close below the upper surface of this. In almost the whole orbito-
temporal region and throughout the ethmoidal region it goes rather
straight beneath the supra-orbital sensory canal to which fine rami issue
from it {rm.ophth.lat, Pis, XXI, XXIII), a fact which fully shows
that it lodged the n. ophthalmicus lateralis. It may, therefore, properly
be called the canalis n. ophthalmici lateralis. In the subsequent descrip-
tion we shall return to it and its importance for the understanding of
the trigemino- facialis ganglionic complex.
Not far distally of the canalis n. ophthalmici lateralis, a second
branch issues from the facialis canal, but on the antero-ventral side.
This branch {c.bucc.lat, Figs. 10, 11), which has almost the same
calibre as the canalis n. ophthalmici lateralis, goes forward and
opens into the trigeminus canal, and, as the trigeminus and facialis
canals lie close to each other at this place, it is rather short. As will
be evident from my subsequent account of the trigemino-facialis gan-
glionic complex, the canal transmitted certain lateralis fibres, viz., those
that formed the n. buccalis lateralis.
A third, wide, but short branch (c.pal1, Figs. 10, 11) is given
off from the facialis canal close to the external opening. This branch
runs forward to the postero-ventro-lateral corner of the orbit, and,
as will be shown below, the conditions are such that it must have been
pierced by the r. palatinus facialis.
It has already been pointed out above, that the fine but pronounced
1This branch was discovered by preparation after the photographs had been
taken and is therefore not shown in the plates.
Macropetalichthyids and other Arthrodires — Stensio 109
groove s.ra, along the ventral border of the external (lateral) surface
of each lateral wall deepens and is transformed into a canal a short
distance anterior to the external opening of the glossopharyngeus canal,
the canal arising in this way {era, Figs. 1, 5, 7; Pis. XXI-XXIII)
being fairly narrow. Not far dorsally of this canal is found the
posterior opening (ju2) of another much wider canal (ju, Figs. 1,
5. 7; Pis. XX-XXIII). These two canals, which we, after their index
letters in the previous figures, may refer to as era, and ju respectively,
go forward within the lateral part of the lateral wall, where they soon
meet and join to a wide sinus (si, text fig. 7; Pis. XXI-XXIII). In
this sinus there may be distinguished a wide, dorsal division correspond-
ing to the canal ju and a narrow, ventral division corresponding to the
canal era. Although very imperfectly separated from one another,
these divisions indicate that the structures traversing the two canals ju
and era, retained their independence and original positions in the
sinus, si.
The ventral division of the sinus si, is continued forward by a
canal which is lettered ecom in the figures (Fig. 7; Pis. XXI,
XXIII) and which runs in an anterior and slightly medial direction
at the transition between the lateral and ventral walls of the region.
It is also noteworthy that this canal (ecom) has a very superficial
position, as it is situated wholly within the external bone layer. After
a short course forward it divides into two branches of about equal
calibre, a lateral one (ecar.ext) and a medial one (ecar.int). The
former of these runs latero-dorsally through the lateral wall and opens
close to and postero-ventrally of the facialis canal (Figs. 1, 7;
Pis. XXI, XXIII), while the latter takes a course forward and slightly
medially to the orbitotemporal region. The former of these branches
is throughout its extension situated within the external bone layer
(Fig. 8), as is also the latter till it reaches the anterior end of
the orbitotemporal region (Fig. 8), where it suddenly leaves the
external bone layer and ascends into the cartilaginous interior of the
ventral wall in the manner that will be described below in the account
of the orbitotemporal region. As we shall find from its course, the
latter branch (ecar.int) must have lodged the arteria carotis interna
and is therefore termed the internal carotid canal. This being the
case, it is easy to understand that the other branch (ecar.ext) must
have been traversed by the arteria carotis externa. (Cf. Allis 1897,
PP- 497-50O; 1908a; 1908b; 1909a, pp. 51-55, 185-187; 1909b; 1911a;
1911b; 1912a, b, c, d; 1914a; 1914b; 1919a, 1922a, pp. 266-268; 1922b;
Allen 1905, pp. 51-62; Danforth 1912, pp. 435-445; Greil, 1913,
1 10 Field Museum of Natural History — Geology, Vol. IV.
PI. LV, fig. 2.) Hence this may properly be termed the external
carotid canal.
The branches of the canal c.com, having had these functions, it is
evident that the canal c.com itself must have lodged the arteria carotis
communis. From the canal c.com, the arteria carotis communis may
have continued a short distance backwards into the ventral division of
the sinus si, there probably, as we shall see, having received the arteria
efferens hyoidea and having become the radix aortae (lateral dorsal
aorta). The radix aortae must then have traversed the remaining pos-
terior part of the ventral division of the sinus si behind this, continuing
through the canal era, and the groove s.ra, to the anterior end of the
haemal groove (hczm). At the anterior end of this it probably met its
fellow of the opposite side and joined with it to the unpaired aorta
dorsalis, which in its backward course occupied the haemal groove.
The efferent arteries from the two or three anterior branchial arches
may probably have emptied into the radix aortae of their side, while
those from the remaining posterior branchial arches probably opened
into the anterior portion of the aorta dorsalis. My view concerning
the course and mutual relations of the arterial trunks mentioned is
further elucidated by the diagrammatic sketch reproduced in Fig. 6,
and as is evident from this the arterial system of the head seems to
fend
c.ophth.sup.V?
cr.'ol
V
ju-i c.hy IX S.ra,
Fig. 5. Macropetalichthys rapheidolabis
Primordial neurocranium in lateral view. Occipital region after specimens
in the American Museum, New York, other regions after a specimen in Field
Museum, Chicago. Bone dark, cartilage dotted. XH-
a.opt, division of the distal part of the opticus canal, probably for the arteria
optica; c.hy, canal for the v. hyoidea; c.ophth.sup.V ?, canal probably for the
r. ophthalmicus superficialis trigemini ; cr.od, crista occipitalis dorsalis ; cr.ol,
crista occipitalis lateralis ; cr.sp, cranio-spinal process ; /wi, anterior opening of
the canal ja. The opening was traversed by the vena mandiburalis ; jui, posterior
opening of the canal ju; na, nasal aperture; orb, orbit; s.ra, groove for the radix
aortae ; vy, canal of doubtful importance ; either for vessel or nerves or for
both ; II, opticus canal ; IX, glossopharyngeus canal ; Xvn, canal for the n. vagus
and the vena cerebralis posterior.
Macropetalichthyids and other Arthrodires — Stensio III
resemble mostly that in Chimcera (Allis 1912a). The arteria carotis
interna will be further treated below in the description of the orbito-
temporal region.
Turning again to the canal ju, we find that anterior to the sinus si
(Fig. 7; Pis. XXI, XXIII) it takes a more lateral direction than
in its posterior part. It opens with its anterior opening (juy) on the
L-— <z.pal
cuc.int
a.coru F^5f \
..-a.effhy
" a "rt" ]^jfeS?N - 'VJU
—v.hy
rSk\^z^
\ J$^<w\ ueffI
i& ^p 1 \
1 1
v.oerebr.jjost ~
•
u-dtors
Fig. 6. Macropetalichthys rapheidolabis
Diagrammatic sketch of the main, arterial and venous trunks of the primordial
neurocranium. Outlines of neurocranium in ventral view. Arteries red, veins
blue. The parts of the different vessels situated in canals or cavities indicated
by transversal bands of color. X/^.
a.com, arteria carotis communis; a.c.ext, arteria carotis externa; a.c.int, arteria
carotis interna; a.eff.hy, arteria efferens hyoidea; a.effJ-a.eff.V, efferent arteries
of the branchial arches I-V ; a.dors, aorta dorsalis ; a.pal, possible palatine branch
of the a. carotis interna ; r.aort, radix aortae ; v.cerebr.post, vena cerebralis pos-
terior; v.hy, vena hyoidea; v.ju, vena jugularis; v.m, vena mandibularis (from
the dorsal parts of the mandibular arch to the jugular vein).
ii2 Field Museum of Natural History — Geology, Vol. IV.
external (lateral) surface of the lateral wall rather close laterally to
the facialis canal. With the part situated anterior of the sinus si, it
joins on the medial side a fairly wide canal jg, which comes from the
postero-latero-ventral corner of the orbit, perforating in its course back-
wards the ventral portion of the postorbital process. This canal, jg,
does not, however, run straight^backwards but postero latero-ventrally
and crosses in its course the distal part of the facialis canal on the
dorsal side. Its position may be understood from PI. XXI, PI. XXII,
PI. XXV, figs, i, 3, and Figs. 6, 7. (In PI. XXIV is seen a section of it
just after it has crossed the facialis canal; in PI. XXV, fig. 1, it is
seen crossing the facialis canal on the dorsal side, and it is also obvious
that the bone layer lining it is continuous proximally with the part of
the external bone layer that covers the posterior surface of the orbit;
PI. XXV, fig. 3, shows the same part of it as the preceding figure but
from behind.)
From the conditions in recent fishes it is not difficult to conclude
what the functions of the canals ju and jg were. Both must have been
traversed by veins. The vein that traversed the canal jg evidently col-
lected the blood from the eye bulb, the eye muscles and probably also
from the anterior parts of the cavum cerebrale cranii. Hence it must
have corresponded to the vena jugularis interna according to the
terminology mostly employed hitherto. Allis has, however, recently
proposed to call this vein simply the jugular vein, a term that I shall
adopt here as I have done in my other works. (Cf. Allen 1905, pp.
78-90, PL I, figs. 1, 5; PL II, figs. 13, 15; PL III, figs. 23-25; Allis
1897, pp. 500-506; 1903, p. 93; 1908b, pp. 219-222; 1909a, pp. 40,
45, 50, 187, etc.; 1914a; 1914b; 1919a, p. 222, and in several other
places throughout the paper; 1922a, pp. 260-264; 1922b, p. 153; Greil
1913, PI. 54, fig. 1 ; PI. 55, figs. 2, 8 (vena capitis lateralis) ; Grosser
1907; Lehn 1918, pp. 365-372; O'Donoghue 1914, pp. 438-443, an-
terior cardinal sinus -f- postorbital sinus; Parker, T. J., 1887, pp. 711-
713; Stensio 1921, p. 178; 1923, pp. 1244, 1260-1268; 1925.)
A vein of a considerable size must have entered the canal ju through
the anterior opening ju±. As far as can be judged, this vein collected
the blood from the palatoquadrate and perhaps also from certain parts
of the ethmoidal region; in other words, it seems to have drained a
large dorsal part of the mandibular arch and nearest surroundings.
Hence it may be properly termed the vena mandibularis.
The vena mandibularis of the fish so defined, after a short passage
backwards within the canal ju, met and joined the jugular vein to a
common large stem which then passed backwards through the dorsal
Macropetalichthyids and other Arthrodires — Stensio 113
division of the sinus si and the posterior part of the canal ju behind
this sinus, then continuing along the external surface of the lateral
wall of the neurocranium received during this latter part of its course
the vena cerebralis posterior as it passed the external opening of the
vagus canal. Despite the fact that the vena mandibularis was much
larger than the vena jugularis, the common stem described, that arose
by its confluence with the latter, must for morphological reasons be
considered as the backward continuation of this, and hence throughout
its extension it must retain the name of jugular vein. The course and
relations of the juglar vein, the mandibular vein and the vena cerebralis
posterior seem to have been as indicated in the diagrammatic sketch
in Fig. 6.
In the dorsal division of the sinus si — thus, after what we have
just found, the division for the jugular vein — there opens on the lateral
side a short but rather wide canal c.hy (Figs. 1, 5, 7; Pis. XX-
XXIII; Pi. XXIV, figs. 1, 3), which leads from the external surface
of the lateral wall of the region, or, to define it more closely, from the
postero-dorsal corner of the anterior part of the posterior field of the
lateral surface of the region. From its course and position we may
conclude with a rather large degree of probability that it was traversed
by a vein. This supposed vein must have come from the hyoid arch
and emptied into the jugular vein, thus corresponding partly to the
sinus hyoideus of Selachians. I call it here the vena hyoidea.
In Selachians the sinus hyoideus is a large vein which runs upward
on the posterior side of the hyoid arch, receiving blood not only from
this arch, but also from the dorsal and middle parts of the mandibular
arch (Parker, T. J., 1887, PI. 35, fig. 10; O'Donoghue 1914, pp. 441-
442), a condition which explains the fact that a mandibular vein is
lacking. In Polypterus a hyoid vein seems also to occur (Allis 1922a,
p. 264), but in Ceratodus (Greil 1913, Pis. 54, 55) and the Loricati
(Allen 1905, pp. 79-80), the dorsal parts of this vein seem to be
lacking. At least in the Loricati the venous blood of the hyoid arch
goes chiefly forward to the mandibular vein, arriving through this into
the vena jugularis. In Ceratodus the conditions in this respect are not
known with certainty. The venous system of other fishes besides those
mentioned, is not known in detail and accordingly nothing can be said
of the veins in the mandibular and hyoid arches there.
The occurrence of both the vena mandibularis and the vena hyoidea
in Macropetalichthys rapheidolabis is, it seems to me, to be considered
a primitive character, since we may presume that the venous system
of fishes originally had a segmental arrangement.
ii4 Field Museum of Natural History — Geology, Vol. IV.
If the interpretation given here of the canal c.hy of Macropetalich-
thys rapheidolabis is correct, and if the vena hyoidea during the passage
to this canal went approximately as it does in Selachians, it is evident
that the dorsal end of the hyoid arch must have articulated against the
lateral wall of the region anterior to the external opening of the canal
c.hy. What this means for views concerning the homologies of
the hyomandibular in Selachians and Teleostomes will be dealt with
below in the account of the visceral skeleton. It has been mentioned
above that the arteria efferens hyoidea ought to have persisted in
M. rapheidolabis, and if so, it probably, as in recent fishes in which
it is found, went upwards along the posterior side of the hyoid
arch and emptied into the anterior part of the radix aortae, which, as
we have seen, was situated here within the sinus si. After leaving the
hyoid arch, the a. efferens in order to reach this sinus must have
entered the external opening of the canal c.hy, as no other foramen
can be found for it. Perhaps it did not, however, traverse the entire
canal c.hy, but took near the proximal end of this a more anterior
course, entering the rather fine canal c.ef (Fig. 7; Pis. XX, XXII),
which, at least on the left side of the Field Museum specimen, runs
from the proximal part of the canal c.hy forward, medially and down-
ward to the anterior end of the sinus si. If well arrived into the sinus
si in this way, the artery must have crossed the jugular vein and passed
downwards to the ventral division of the sinus, joining there the radix
aortae at the transition to the arteria carotis communis. It may, how-
ever, be possible that the canal c.ef, instead of being traversed by the
arteria efferens hyoidea, as here supposed, transmitted the truncus
hyoideomandibularis facialis, and if this was the case, the a. efferens
hyoidea must of course have traversed the entire canal c.hy, and have
entered the sinus si much farther back. (Allen 1905, pp. 44-62 ; Allis
1908a, b; 1909a, pp. 51-52, 183-184; 1909b; 1911a, b; 1912a, b, c, d;
1922a, pp. 264-268; Danforth 1912, pp. 435-445.)
In most fishes in which there is a canal for the jugular vein in
the lateral wall of the neurocranium, the jugular vein is generally
during its passage through this canal accompanied for some distance
by the truncus hyoideomandibularis, which enters the canal either from
in front through the anterior opening or from the medial side. In
the former case the truncus hyoidemandibularis has its exit through
the cranial wall anterior to, in the latter case into the canal for the
jugular vein. (Cf. Allis 1897, pp. 492-497; 1903, pp. 87-95, 2^6;
1908b, pp. 219-222; 1909a, pp. 44-45; 1911a, p. 291; 1914a; 1914b;
Macropetalichthyids and other Arthrodires — StensiS 115
19x9a; 1922a, pp. 263-264; 1922b; Lehn 1918, pp. 363-372; Pollard
1892a, PI. 30, figs. 27-30; Stensio 1921, p. 178; 1923, pp. 1244, 1261,
1262-1268; 1925; etc.)
Concerning M. rapheidolabis , we cannot say at present whether the
truncus hyoidemandibularis after its exit from the facialis canal defin-
itely left the cranial wall or whether it again entered the cranial wall
accompanying the vena jugularis for some distance. If the former of
these alternatives were true, the truncus hyoidemandibularis would have
left the cranial wall rather far anterior of the hyoid arch and would
thus have reached this from in front in the same way as in Selachians.
(Cf. Ruge 1897, Figs. 3, 12, 13, 14; Goodrich 1909, Fig. 44; Allis
1915; 1918b; cf. also van Withe 1882, PI. 15, figs. 1, 3, 4, 6; etc.)
If, on the other hand, the latter were true, it is at once evident that the
truncus hyoideomandibularis cannot have entered the anterior opening
of the canal jg for the jugular vein, (as we have found, this opening
is situated in the orbit, while the external opening of the facialis canal
lies on the lateral surface of the lateral wall behind the orbit), but
would, together with the vena mandibularis, have passed through the
anterior opening (/*j) of the canal ju. In other words, the truncus
hyoideomandibularis would, with this alternative, have reached the
vena jugularis in the anterior part of the canal ju, after traversing the
part of this canal situated anterior to the canal jg.
Assuming that it did so, it may during its course further back-
ward either have traversed the entire dorsal division of the sinus si
and behind this the posterior part of the canal ju, or may soon after its
entrance in the sinus si have turned latero-dorsally, entering and
traversing the canal c.ef and the distal portion of the canal c.hy. If
it took the former of these courses it would have left the cranial wall
considerably behind its arch, the hyoid arch, which, as we have seen,
articulated against the lateral wall of the region anterior to the external
opening of the canal c.hy. It would thus have gone a considerably
roundabout way to its destination, which seems rather improbable. If,
on the contrary, it passed from the sinus si through the canal c.ef and
the canal c.hy, it would, of course, have had its exit from the cranial
wall in closer relation to its arch, although still behind this.
The conditions on the external surface of the lateral wall of the
region between and around the external opening of the facialis canal
and the anterior opening of the canal ju, would, perhaps, if they were
known in detail, enable us to decide with considerable degree of
probability what course the truncus hyoideomandibularis took after
n6 Field Museum of Natural History — Geology, Vol. IV.
leaving the facialis canal1. But, unfortunately, the external surface of
the wall is not sufficiently well preserved to enable me to determine
this in any of the specimens so far investigated by me.
In dealing with the trigeminus and facialis canals below we shall
see that the r. oticus lateralis must have originated in the proxifnal part
of the facialis canal, which it ""left either through the external opening,
or through a canal of its own. In the latter case it would have issued
from the dorsal side of the distal part of the facialis canal. A few
fine canals, which were traversed by rami from the r. oticus lateralis to a
sensory canal portion in the dermal cranial roof are seen in the Field
Museum specimen (in, Pis. XX, XXII). These fine canals could,
however, not be traced downwards to their origin, and, therefore, we
cannot say whether they come from a larger canal within the cranial
wall or whether each of them went independently down and opened on
the lateral surface of the lateral wall.
Labyrinth Cavity
The labyrinth cavity is completely preserved only in the Field
Museum specimen. Traces of it are, however, seen also in one of the
New York specimens (PL XXVII). It (lab.cav, Fig. 4; PL XX; PL
XXVI, figs. 3, 4; PI. XXVII) is large and, as pointed out, lined
throughout by a thin layer of perichondral bone, the labyrinth bone
layer. As has also been mentioned, it is completely separated from the
cavum cerebrale cranii by a thick septum (PL XX) which consisted
of cartilage lined by the labyrinth bone layer on the labyrinth side and
by the internal bone layer on the cerebral side.
The division for the sacculus (sac, PL XXVI, fig. 3) is not well
exposed, but, as far as can be judged, it cannot have been very large.
It is not distinctly separated from the remaining divisions of the
cavity. From its dorsal part a canal (d.end, Fig. 4; PL XX;
PL XXVI, fig. 3) issues in a dorso-postero-lateral direction to the
dorsal surface of the primordial neurocranium. This canal seems,
however, to end some distance below the dermal bones of the cranial
roof, a fact which probably indicates that the cartilage at this place
did not reach quite up to the dermal bones, i.e., that there probably
was a shallow fossa into which the canal had its dorsal opening
1 The presence of a groove between the two openings would of course have
indicated that the r. hyoideomandibularis after its exit from the facialis canal
went backward along the lateral surface and entered the canal ju through the
anterior opening j'ih. The absence of such a groove would of course point in
the opposite direction. If the two openings should be found situated in a com-
mon pit this must probably also indicate that the tr. hyoideomandibularis entered
the canal ju through the anterior opening jui.
Macropetalichthyids and other Arthrodires — Stensio 117
(Fig. 3; Pis. XIX, XX). From its course and relations it is evi-
dent that the canal in question must have lodged the ductus endo-
lymphaticus, which, accordingly, opened on the dorsal surface of the
primordial neurocranium in a fossa endolymphatica as in Selachians
(Retzius 1881).
Slightly postero-ventrally of the canal for the ductus endolym-
phaticus, another, but much wider canal, issued from the division for
the sacciilus. This canal (v.lb, Figs. 10, 11 ; PI. XX; PI. XXIV,
fig. 2; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2, 3), which runs back-
ward and somewhat dorsally to the dorsal division of the vagus canal,
has already been described in another connection, and, as pointed out
there, it probably transmitted a large vein, that drained the whole, or
at least the main part, of the labyrinth.
The division for the utriculus (utr, PI. XX; PI. XXVI, figs. 3, 4)
appears to be relatively very large, but as it is not distinctly marked
off from the other divisions, its size may, perhaps, be 'over-estimated.
As is seen from the plates, its largest extension is from above down-
VII c.car.ext
19 \ !
Fig. 7. Macropetalichthys rapheidolabis
Diagrammatic sketch showing the position of certain canals in the lateral
wall of the labyrinth region. Certain ventral parts of the lateral wall removed.
Bone layers indicated with shading and when in cross section with black lines.
Cartilage dotted. XH-
c.com, canal for the arteria carotis communis ; c.ef, canal perhaps for the
arteria efferens hyoidea ; c.hy, canal for the vena hyoidea ; era, canal for the
radix aortse behind the sinus si: jg, canal for the vena jugularis through the
postorbital process; ju, canal behind jg, for the vena jugularis, anterior of jg,
for the vena mandibularis jui, pit, anterior and posterior openings of the canal ju ;
si, sinus arisen by confluence of the canals ju and era; s.ra, groove for the
radix aorta (lateral dorsal aorta); VII, canal for the n. facialis; IX, canal for
the n. glossopharyngeus.
n8 Field Museum of Natural History — Geology, Vol. IV.
wards and forwards, which must mean that the utriculus did not have
a horizontal position, but was inclined in the same way as in Selachians
(cf. Retzius 1881). Just at the antero-ventral end, there is, immedi-
ately anterior of the place of entrance of the acusticus canal (VIII)
a rather pronounced bulge (ru, PI. XXVI, fig. 3), which, to judge
from its position, probably contained the recessus utriculi. Immedi-
ately in front of this bulge lies the proximal portion of the facialis
canal (VII).
The divisions for the semicircular canals are very imperfectly
known, which is due partly to the fact that they could not be laid
bare without danger to other important structures, and partly to the
circumstance that they probably are not well differentiated from the
other parts of the labyrinth cavity. It seems, however, not unlikely
that the part designated with the letters c.sem.ant, in PI. XX and PI.
XXVI, figs. 3, 4, was occupied by the canalis semicircularis anterior,
and if this is true the canalis semicircularis anterior would have had
a position almost transversal to the longitudinal axis of the head. This
position would, as is easily understood, have been caused by the back-
ward shifting of the orbit. What parts of the divisions for the other
two semicircular canals are represented by the fragments denoted by
c.sem in PI. XXI and PI. XXVI, fig. 4, is difficult to say with cer-
tainty, but at least what is seen in PI. XXI seems most probably to
be a part of the division for the canalis semicircularis externus, which
thus would have been situated as in Elasmobranchs.
The n. acusticus, as is seen from the position of its canal (VIII),
enters the labyrinth cavity at the very bottom, as in certain Elasmo-
branchs (Scyllicum, Raja, cf. Retzius 1881), and is rather short.
The glossopharyngeus, as has already been mentioned, traversed the
postero-ventral parts of the labyrinth cavity. The facialis went first
for some distance along the antero-ventro-medial part of the labyrinth
cavity, then turned laterally and passed along the antero-ventral part
of the same cavity between it and the orbit. (PI. XXV, figs. 1, 3;
PI. XXVI, figs. 3, 4.)
From the description now given of the labyrinth cavity it seems
rather probable that this was larger than the membranous labyrinth en-
closed in it, a condition which makes it difficult to conclude anything
with certainty about the finer details of the latter. But what is, how-
ever, known so far of the larger divisions of the labyrinth cavity,
indicates beyond doubt, as we have seen, that the membranous labyrinth,
at least in certain of its general characters, was of the Elasmobranchian
type.
Macropetalichthyids and other Arthrodires — Stensio 119
A more detailed knowledge of the labyrinth cavity would seem evi-
dently to be of great importance for the understanding of the affinites
of Macropetalichthys to other fishes, and it is therefore to be hoped
that new material, fit for a detailed investigation of the labyrinth
cavity, will soon be found.
Orbitotemporal Region
The orbitotemporal region (Figs. 1, 3, 5, 8; Pis. XIX-XXIII; PI.
XXIV, fig. 1 ; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2) is strikingly
short and broad, its breadth being almost three times as great as its
length. The breadth is, however, much greater at the bottom of the re-
gion than higher up, a condition which is caused by the presence of the
very extensive floors for the orbits (Figs. 3, 8; Pis. XIX, XXI, XXIII).
The height is greatest at the posterior end of the region, where it is
about equal to the length and a third of the breadth, decreasing slowly
and gradually forwards throughout the region. On account of the orbital
floors, the region is, at the bottom, not marked off from the adjacent
regions, otherwise being, however, considerably narrower than these.
The orbitae are rather small and lie far apart. While in Arthrodires
in general they are situated fairly far forward, and have the entrances
facing almost straight laterally, they have here shifted a certain dis-
tance in a postero-medial direction and have their entrances directed
much upwards (Figs. 3, 5, 8; Pis. XIX, XX; PI. XXIV, fig. 1).
On account of the shifting, they are much more completely bounded
by the primordial neurocranium than is generally the case in fishes,
for, in fact, they have not only a medial, a posterior and an anterior,
but, in addition, a ventral and to a large extent a lateral wall formed
by this. The ventral wall, which has been already referred to as the
orbital floor, is a rather thin plate which is continuous laterally and
posteriorly with the postorbital process, medially with the ventral part
of the interorbital wall and anteriorly with the ethmoidal region. It
consists of cartilage covered both on the upper (orbital) and lower
surfaces by the external bone layer. The lateral wall, as far as it is
present, is formed by the large postorbital process, which reaches so
far forward that it leaves only a rather limited opening between its
anterior end and the ethmoidal region (Figs. 3, 5; Pis. XIX, XX;
PI. XXIV, fig. 1). A transverse section through the posterior half of
the region that shows the relations between the different walls of the
orbit at this place as well as the shape of the orbit itself is seen in
Figure 8. The orbital entrance is of an elliptical shape, with the
120 Field Museum of Natural History — Geology, Vol. IV.
largest axis directed so as to converge a little towards that of the other
side in a postero-dorso-medial direction (Fig. 3; Pis. XIX, XX;
PI. XXIV, fig. 1).
For detailed description we may consider the region to be com-
posed of four walls — a ventral, a, dorsal, and a right and a left lateral
— all of which, of course, are continuous with the correspondingly
situated walls in the adjacent regions.
The ventral wall (Figs. 1, 5, 8; Pis. XXI, XXII, XXIII; PI.
XXV, figs. 1, 2; PI. XXVI, figs. 1, 2) is characterized in the first
place by its very considerable breadth, which remains almost unchanged
throughout the region. As a whole it is thin, and especially is this
the case in the lateral (orbital floors) and median parts. Like the
corresponding walls in the adjacent regions, it is covered by the inter-
nal bone layer dorsally and the external bone layer ventrally and is
curved so that the ventral surface is concave in transversal direction.
It has no fenestra hypophyseus or fenestration whatever and the fossa
hypophyseus is only very slightly developed on its dorsal (cerebral)
side.
The canal for the internal carotid artery of each side (c.car.int,
Fig. 8; Pis. XXI-XXIII; PI. XXV, fig. 2) enters the ventral wall
of the region from behind and perforates the lateral part of this that
forms the orbital floor. Its direction is forward and a little medially
and, in the posterior larger part of its course, it is here, as in the
labyrinth region, situated entirely within the external bone layer, thus
very superficially. In the anterior part of the region it penetrates
deeper into the wall, curving postero-dorso-medially. With this latter
course it arrives up and into the lateral wall of the region, where it
finally opens into the opticus canal (PI. XXIII; PI. XXV, fig. 2), its
opening into this being situated on the anterior side close to the orbit.
Just at the place where it curves from the anterior to the postero-
dorso-medial direction, it lies somewhat anterior of the orbit, thus
really in the basal part of the antorbital process, a fact which is cer-
tainly explained by the backward shifting of the orbits. We shall have
the opportunity of returning to it again in connection with the de-
scription of the opticus canal.
The orbital floor of each side is perforated by a second canal (c.pal2,
Pis. XXI, XXII) which begins in the orbit and runs antero-ventrally
through the floor, opening into the anterior portion of the suborbital
part of the internal carotid canal. At the place where the latter canal
curves dorso-postero-medially it has an opening (c.pal3, Fig. 1),
leading to the ventral surface of the neurocranium, and from this
Macropetalichthyids and other Arthrodires — Stensio 121
opening a shallow groove (s. pal, Fig. i) goes forward. As we
shall see from the description of the trigemino-facialis ganglionic
complex below, it is not difficult to conclude that the canal c.pal2 was
traversed by the r. palatinus facialis, which passed down to the anterior
portion of the suborbital part of the internal carotid canal, then left
this and arrived at the ventral side of the neurocranium by the open-
ing c.pd3.
The dorsal wall (Figs. 3, 8; Pis. XIX, XX; PI. XXIV, fig. 1;
PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2) is broad and short like the
ventral one, but on the other hand considerably thicker. As far as one
can judge from its appearance in the fossil it had no fontanelle, and,
as already ^pointed out, it is devoid of the external bone layer. Its
upper (external) surface is slightly convex in transversal direction
and provided with grooves for the ridges on the lower side of the
dermal bones in which the sensory canals are enclosed. In its anterior
part it is, as already mentioned above, traversed by the canal for the
n. ophthalmicus lateralis, which arrives there from below from the
lateral wall. Otherwise there are in it only several very fine canals,
the most important of which issue from the canal for the n.ophthalmi-
cus lateralis and pass upwards to the supraorbital sensory canal.
Finally, the lateral walls (Figs. 1, 5, 8; Pis. XXII-XXIV; PI.
XXV, figs. 1, 2; PI. XXVI, figs. 1, 2), although thick, are by no means
as thick as those in the labyrinth region. Their thinnest place is situated
around the opticus canal (II). They are covered both by the internal
and external bone layers. Their external surface is concave both in
dorso-ventral and antero-caudal direction. Their internal surface has
a rather complicated appearance, as may be understood, from the figures
quoted and as will be further dealt with below.
Each lateral wall is perforated by several canals. We begin here
with the most posterior one, the trigeminus canal, proceeding forward
from this.
The trigeminus canal (V, V2,3, Figs. 10, 11; PI. XXIII; PI.
XXV, figs. 1, 2; PI. XXVI, figs. 1, 2) leaves the cavum cerebrale
cranii a short distance anterior to the facialis canal and, as pointed out
above, its internal opening has nothing to do with the acustico- facialis
recess. It runs antero-laterally and somewhat ventrally to the postero-
ventro-medial corner of the orbit, and, on account of the fact that the
cranial wall is much thinner at this place than immediately behind the
orbit, it naturally becomes considerably shorter than the facialis canal.
The external opening, as is easily understood, is situated rather far
medially of that of the facialis canal.
122 Field Museum of Natural History — Geology, Vol. IV.
In the trigeminus canal there may properly be distinguished two
parts, a proximal short and wide one (V) and a distal rather long
and narrower one (V2,3), the former of which actually forms a recess
on the lateral side of the cranial wall, a recess which may be called
the trigeminus recess.
From the antero-dorsal part of the trigeminus recess, as thus de-
fined, there issues a rather wide canal (c.ophth.sup.V) which runs for-
ward and a little upward, crossing the canal for the n. ophthalmicus
lateralis (c.ophth.lat) on the ventro-medial side and opening into the
postero-ventro-medial part of the orbit (PI. XXIII; PI. XXV, fig. 2;
PI. XXVI, figs. 1, 2). Slightly ventrally of this canal a second, but
considerably narrower one, (c.ophth.prof), leaves the trigeminus re-
cess, and taking its course ventro-medially of the canal for the n. oph-
thalmicus lateralis, goes almost straight forward to the orbit, where it
opens somewhat postero-ventrally of the former.
With the knowledge we now possess of the conditions in recent
fishes, it is easy to conclude what the functions of the canals just de-
scribed were, and, in addition, to make out to a certain degree how
the trigemino-facialis ganglionic complex and its roots were developed.
For comparison the reader may be referred to the works of Herrick
(1899; 1900; 1901), Johnston (1898; 1901), Kingsbury (1897),
Allis (1897; 1901 ; 1903; 1909a; 1918c; 1922), Landacre (1916),
Norris and Hughe (1920) and Strong (1895), etc.
As mentioned in another connection, it is fully evident that the
canal designated by the letters c.opht.lat transmitted the n. ophthalmicus
lateralis (cf. above) and as this canal does not communicate with the
trigeminus canal or the fine canals issuing from the latter, we may also
conclude with full certainty that it did not give passage to any general
cutaneous fibres. These fibres to the orbit or dorsal side of the neu-
rocranium must therefore undoubtedly have emerged into the orbit
through the canals c.ophth.prof and c.ophth.sup.V, and it is easy to
understand that those of them which passed through the canal c.ophth.
prof formed the r. ophthalmicus profundus trigemini, or a somewhat
equivalent nerve, while the others which passed through the canal
c.ophth.sup.V formed the r. ophthalmicus superficialis trigemini. In
other words, the general cutaneous fibres which formed the r. ophthal-
micus profundus were transmitted to the orbit through the canal c.ophth.-
prof, and those which formed the r. ophthalmicus superficialis trigemini
through the canal c.ophth.sup.V , while the lateralis fibres which formed
the n. ophthalmicus lateralis never entered the orbit but passed in the
interorbital wall through a canal of their own which is given off from
Macropetalichthyids and other Arthrodires — Stensio 123
the facialis canal and has no connection with the trigeminus canal- (in-
cluding the trigeminus recess). A separate course of the r. ophthalmi-
cus lateralis and r. ophthalmicus profundus trigemini is, besides in the
form now under consideration, found in Acipenscr, Polyodon (Stensio
1925), Saurichtliys (Stensio 1925) and Siluroids (Workman 1900,
pp. 403-407; Herrick 1901, pp. 201-203).
The r. ophthalmicus superficialis trigemini and the r. ophthalmicus
profundus trigemini having issued from the trigeminus recess — the
proximal part of the trigeminus canal — it is fully evident that this
recess must have contained at least the main parts of the ganglion
gasseri and the profundus ganglion, i.e., the main parts of the ganglia
of the general cutaneous fibres. Only small parts of these ganglia
may perhaps have reached into the cavum cerebrale cranii. From the
origin of the r. ophthalmicus superficialis and the r. ophthalmicus pro-
fundus trigemini, we can further conclude that the distal narrower
part of the trigeminus canal (V2,3) transmitted the r. maxillaris and
the r. mandibularis trigemini after their origin from the ganglion
gasseri. That these two latter rami were accompanied by certain
lateralis fibres and perhaps also by certain communis fibres, we shall see
from the subsequent account.
During their passage through the cranial wall, the trigeminus roots
proper (i.e., the general cutaneous roots and the motor root) of recent
fishes are, as a rule, accompanied by a lateralis root, which leaves the
medulla oblongata postero-dorsally of the trigeminus root proper run-
ning outward postero-dorsally or dorsally of these, and having its
ganglion dorsally or postero-dorsally of the ganglion gasseri. From
the ganglion of this lateralis root the n. ophthalmicus lateralis and
n. buccalis lateralis take their origin.
In Macropetalichthys rapheidolabis, in which the canal for the
n. ophthalmicus lateralis issues from the facialis canal and in which,
as we have seen, there is no communication between the proximal
parts of the latter canal and the trigeminus canal, the lateralis root
which gave rise to the n. ophthalmicus lateralis and the n. buccalis
lateralis must evidently have left the cranial cavity through the facialis
canal, traversing the proximal part of this canal and having its ganglion
there. The n. buccalis lateralis thus also arose within the facialis canal.
In order to reach the trigeminus canal, the n. buccalis lateralis in
M. rapheidolabis must, soon after its origin, have turned forward,
traversing the short canal c.buc.lat, described above (Figs. 10, 11),
which leaves the facialis canal slightly distally and somewhat ven-
trally of the canal for the n. ophthalmicus lateralis and goes to the
124 Field Museum of Natural History — Geology, Vol. IV.
distal narrow part (V2,3) of the trigeminus canal. The n. buccalis
lateralis after arriving into this part of the trigeminus canal, probably
joined more or less intimately the r. maxillaris trigemini and passed
distally with this in the normal way.
From the considerations given the following is evident in M.
rapheidolabis concerning the trigemino-facialis ganglionic complex and
its roots : i . Into the trigeminus recess there entered only the general
cutaneous roots and the viscero-motor root of the mandibular arch.
2. The ganglion gasseri and the profundus ganglion must have been
situated in a proximal wide part of the trigeminus canal, called
here the trigeminus recess. 3. Into the facialis canal there entered all
the lateralis fibres and all the communis fibres of the ganglionic com-
plex and, further, the viscero-motor root of the hyoid arch. 4. The
ganglia of the fibres entering the facialis canal, i.e., in this case the
ganglion geniculi, the ganglion of the dorsal lateralis root and the
ganglion of the ventral lateralis root, must have been situated in the
proximal part of this canal and partly, perhaps, in the acustico-facialis
recess.
The lateralis ganglion of the dorsal lateralis root (from which the
n. ophthalmicus lateralis and n. buccalis lateralis arose) must in the
facialis canal have had its position antero-dorsally of the two other
ganglia, the ganglion geniculi and the ganglion of the ventral lateralis
root. Although separated from the ganglion gasseri by a rather thick
wall, it thus had, as far as can be judged, about its normal position in
relation to this, and the fact that it was situated in the facialis canal
is therefore of hardly any importance from a morphological point of
view. Similar arrangements would certainly arise in many recent forms
if the septa of connective tissue that there more or less completely
separate the ganglia in question and their roots from each other
developed more strongly and chondrified or ossified. Moreover, we
find in certain recent forms that ail the roots of the trigemino-facialis
ganglionis complex may run close together through a single canal in
the cranial wall, and that their ganglionic formations may be very
intimately connected with each other. Thus the wall between the
trigeminus and facialis canals in certain cases is reduced. That varia-
tions occur within certain bounds in the trigemino-facialis ganglionic
complex of fishes is accordingly not so remarkable.
Whether in M. rapheidolabis any general cutaneous fibres went
backward to the facialis branches and any communis fibres forward
to the trigeminus branches it is not possible to say with certainty from
the conditions in this fossil, but it is not impossible that this was the
Macropetalichthyids and other Arthrodires — Stensio 125
case, for these fibres can very well be thought to have traversed the
canal c.buc.Iat. Possible communis fibres to the n. ophthalmicus lateralis
must have entered the canal for this nerve directly from the facialis
canal.
The r. oticus lateralis, which, as is known, is a branch from the
n. buccalis lateralis, certainly arose within the facialis. canal, probably
just as the n. buccalis lateralis on its way forward entered the canal
c.buc.Iat. Whether the r. oticus lateralis then traversed the entire re-
maining part of the facialis canal or sooner or later perforated its
dorsal wall, entering a canal of its own, it is impossible to decide at
present.
Apart from the r. oticus facialis, the distal part of the facialis
canal (distally of the point of origin of the ophthalmicus lateralis
canal) transmitted communis fibres — the viscero-motor fibres — for the
hyoid arch, lateralis fibres for the n. mandibularis externus and pos-
sibly also a few general cutaneous fibres. All these fibres formed to-
gether a large facialis stem which, soon after its origin from the
ganglia within the canal, must have branched into the r. palatinus and
the truncus hyoidemandibularis.
The r. palatinus, after its origin in the facialis canal, must first
have turned forward and traversed the canal c.palx which leads from
the facialis canal to the orbit (cf. above). Well within the orbit the
nerve continued forward for some distance in a groove on the orbital
floor, then penetrated downwards in the floor through the canal
c.pal2 to the suborbital part of the internal carotid canal. In the last
mentioned canal it went only a short distance forward, leaving it
through the opening c.pal3 and arriving at the lower surface of the
primordial neurocranium. When emerging from the opening c.pal3
it was perhaps accompanied by a fine branch from the arteria carotis
interna (Cf. text fig. 6).
The correctness of the course of the r. palatinus facialis of the
fish as sketched here is proved by the conditions in recent fishes, in
which the nerve in question also runs in a similar way and also often
more or less closely accompanies the a. carotis interna for a correspond-
ing distance. (Allis 1897, pp. 498, 685, PI. XXXVI, fig. 61 ; PI.
XXXVII, figs. 62, 63; 1901, p. 182; 1909a, pp. 43-55; 88, 185-186;
1911a, p. 290; 1919a; 1922a, pp. 280-281; 1923, p. 213; Allen 1905,
pp. 51-56; Greil 1913, PI. LV, fig. 2; etc.; cf. also Stensio 1923,
pp. 1241-1248; 1925.)
If in M. raphcidolabis there was an anastomosis between the r.
palatinus facialis and the r. palatinus glossopharyngei, this anasto-
126 Field Museum of Natural History — Geology, Vol. IV.
mosis must have been situated in the anterior suborbital part of the
internal carotid canal and the r. palatinus glossopharyngei must have
arrived there through the canal era, the sinus si, and the canal c.cotn
(Cf. Allis 1897, Pis. XXXVI, fig. 61; PI. XXXVII, figs. 62, 63;
1909a, p. 186; 1911a, p. 289-290). The canal for the n. ophthalmicus
lateralis has already been dealt/ with, and I call attention here only
to the fact that from its origin in the lateral wall of the labyrinth
region it goes forward and upward to the lateral wall of the orbito-
temporal region, . from which it soon arrives up and into the dorsal
wall, continuing into this through the entire ethmoidal region. It gives
off several rami in dorsal direction to the sensory canal situated dor-
sally of it, i.e., the supraorbital sensory canal.
After having dealt with the trigeminus and facialis nerves and
their roots and ganglionic complex, we can now understand better than
before the relation between these structures and the jugular vein, and
that my view of the course of the latter ought to be correct. As I
suppose this vein to have run, it must, after its origin in the orbit,
have passed backward below the r. maxillaris and the r. mandibularis
trigemini. During its passage through the basal part of the postorbital
process it then crossed the facialis canal on the dorsal side, thus pass-
ing ventrally of the trigeminus and dorsally of the facialis. Although
it ran rather far laterally of the trigemino-facialis ganglionic complex
and did not, as in most recent forms, traverse this, it had, however,
obviously retained its normal relations to the trigeminus and facialis
nerves. (Cf. Allis 1922a, pp. 260-264.)
Neither for the n. trochlearis nor for the n. abducens have canals
so far been found. The trochlearis canal was probably very narrow,
and may, therefore, have been overlooked, especially as the medial
wall of the orbit is very imperfectly known in detail at the place where
this canal ought to have been situated. That the abducens canal is
not found may perhaps be explained by concluding that there was in
reality no independent canal for the n. abducens, for this nerve may
perhaps have emerged into the orbit more or less closely associated
with the r. maxillaris and r. mandibularis trigemini, as it does in
certain recent forms, for instance, certain Selachians. (Allis 1901,
p. 132; etc.)
A rather wide canal for the n. oculomotorius opens into the orbit
somewhat in front of the canals for the n. ophthalmicus profundus and
the r. ophthalmicus superficialis (V, Figs. 8, 10, 11; PI. XXIII; PI.
XXV, fig. 2; PI. XXVI, figs. 1, 2). A short distance in front of that
canal, the wide opticus canal (II, Figs. 5, 10, 11 ; PI. XXIII ; PI. XXV,
Macropetalichthyids and other Arthrodires — StensiO 127
fig. 2) pierces the lateral wall in an anterolateral direction, opening
into the anterior half of the orbit not far above the floor. In its most
distal (orbital) part it receives on the anterior side the canal for the
arteria carotis interna (c.car.int), which, during its way upwards from
the ventral wall, makes a long curve forward, so that for a short
distance it even traverses a posterior part of the ethmoidal region.
We thus find that the arteria carotis externa entered the distal
part of the opticus canal. From there it evidently must have traversed
the canal in medial direction to reach the cavum cerebrale. But just
as it turned medially to do this it probably gave off the branch known
as the arteria optica, for the external opening of the opticus canal
has on its anterior side distally of the confluence with the internal
carotid canal a distinct emargination (a.opt), which cannot have had
any other function than to transmit a vessel. (Cf. Allen 1905, pp. 56-
57; Allis 1897, p. 498; 1908a, p. 258; 1908b, p. 223; 1909a, p. 186;
1911a, p. 290; 1911b, p. 518; 1912a; 1912b, pp. 489-490; 1912c, p.
586; 1914a; 1922a, p. 168; 1923, PI. XIX; Danforth 1912, pp. 442-
445 etc.)1
The way in which the arteria carotis interna of the fish entered the
cavum cerebrale may perhaps at first glance appear rather strange, as
this artery in fishes generally pierces the ventral wall of the primordial
neurocranium so that it arrives immediately into the cavum cerebrale
and not first into the opticus canal. But a closer study makes it clear
that the difference is really not so important, for in certain forms,
e.g., Saurichthys (Stensio 1925), the canal for the internal carotid
opens either in the proximal part of the opticus canal or at the tran-
sition between this and the cavum cerebrale cranii. We have there a
stage intermediate between the one in Macropetalichthys raphcidolabis
just described and the one generally occurring in fishes. In this con-
nection it is also noteworthy that the arteria carotis interna in Polyptcrus
(Allis 1908b, loc.cit; 1922a, loc.cit; Lehn 1918, p. 380) runs exter-
nally of the interorbital wall so far forward that it enters the cavum
cerebrale either together with or close behind the n. opticus, and that
in Elasmobranchs the same artery within the lateral wall of the neuro-
cranium goes a long distance forward and upward in direction towards
the opticus canal before entering the cavum cerebrale. (Allis 1912b,
loc.cit; 1914a, pp. 229-230, 236, 238, etc.)
'Another interpretation is possible, viz. : — that the arteria pseudobranchialis
efferens reached the a. carotis interna just in the opticus canal, and that the
a. ophthalmica magna was given off to the orbit through the emargination on
the anterior side of the opticus canal. This emargination would consequently in
this case have been traversed by both the a. ophthalmica magna and the a. pseudo-
branchialis efferens.
128 Field Museum of Natural History — Geology, Vol. IV.
Whether a canal for the arteria pseudobranchialis efferens was
present or not in the lateral wall could not be decided from the material
available, and it is therefore not possible to conclude anything with
certainty concerning the relations between the arteria pseudobranchialis
efferens and the arteria carotis interna. (Cf. the footnote on the pre-
ceding page.)
As is seen from the description given of the trigeminus and facialis
canals, there is no real trigemino-facialis chamber in Macropetalichthys
raphcidolabis.
A posterior myodome also does not occur in M. rapheidolabis, and
it does not seem probable that there was an anterior one either. A
satisfactory decision in the latter respect is, however, not as yet pos-
sible, since the conditions of the anterior wall of the orbit are very
imperfectly known.
Since the postero-ventro-medial corner of the orbit as well as the
postero- ventral part of the interorbital wall were partly covered by
matrix, which could not be removed without hurting the fossil as a
specimen, the pituitary canal (canal for the pituitary vein) could not
be observed, but I do not doubt that it actually exists, for it is, as
far as I know, without exception found in all fishes, although in those
forms which have a posterior myodome it is a part of this and has
been much widened and transformed. (Cf. Allis 1909a, pp. 183-208;
cophih.lat
C-ophth.Su.pV *«
cophthprqf
o.ca-r.ext cca.T-.ini
Fig. 8. Macropetalichthys rapheidolabis
Transverse section through the posterior part of the orbitotemporal region
and the antero-lateral parts (postorbital processes) of the labyrinth region. Layers
of substitution bone drawn with black lines, dermal bones with vertical striation.
Cartilage dotted. X-K-
Li, L2, Mi, Pi, dermal bones of the cranial roof (boundaries approximately
drawn); c.car.cxt, canal for the arteria carotis externa; c. car. hit, canal for the
arteria carotis interna; c.ophth.lat, canal for the n. ophthalmicus lateralis; c.
ophth.prof, canal for the r. ophthalmicus profundus trigemini; c.ophth.sup.V ,
canal for the r. ophthalmicus superficialis trigemini ; cv, cavum cerebrale cranii ;
ifc, infraorbital sensory canal ; jg, canal for the vena jugularis through the
postorbital process (somewhat diagrammatically drawn as it really would not
be visible to this extent on a single transverse section) ; ju, canal in the part
seen in the section for the vena mandibulars, behind this place for the vena
jugularis; orb, orbit; soc, supraorbital sensory canal; III, canal for the n.
oculomotorius ; VII, facialis canal.
Macropetalichthyids and other Arthrodires — Stensio 129
1911a, p. 291; 1914a; 1918a; 1919a; 1922a, p. 228; 1922b; Gegenbaur
1872, pp. 75-79, canalis transversus Allen 1905, p. 81 ; O'Donoghue
1914, pp. 440-441; Stensio 1921, pp. 62, 177-180; 1922, p. 179;
1923, pp. 1244, 1262-1268; 1925, Lehn 1918, p. 380.)
Owing to reasons similar to those which prevented study of the
pituitary canal, it was not possible to see any canal for the vena cere-
bralis anterior, which in fishes usually perforates the upper part of
the interorbital wall and goes to the orbit. (Cf. Allis 1897, PI. 21,
figs. 9-1 1, acvfr; 1903, PI. 4, fig. 9, acvfr; 1909a, p. 39; 1922a, pp.
261-262 Lehn 1918, p. 380, figs. 3, 4; Stensio 1921, pp. 168-169;
1923, pp. 1246-1247; 1925.)
Ethmoidal Region
As compared with the labyrinth and orbito-temporal regions the
ethmoidal region is fairly long, its breadth being, however, also con-
siderable, and even so considerable that it exceeds the length. On
account of the presence of the extensive orbital bottom, the region is
not marked off from the orbitotemporal region ventrally. Higher up
above the bottom of the orbit it is, however, much broader than the
latter region, forming the anterior wall of the orbits and projecting on
each side as a preorbital process in the normal way (Figs. 1, 3, 5; Pis.
XIX-XXI; PI. XXIV, fig. 1; PI. XXV, figs. 1, 2).
Both the height and breadth of the region decrease anteriorly, the
latter, however, proportionally somewhat more than the former. The
anterior end is imperfectly preserved, but it is probable that it was
truncated, and, to judge from the conditions in Epipetalichthys de-
scribed below, it is likely that it was, in addition, a little concave.
Close to the anterior end of the region there projects from the
basal parts of each side a strong, broad process (olf, Figs. 1, 3, 5;
Pis. XXI-XXIII; PI. XXIV, fig. 1) in a lateral and somewhat ven-
tral direction. This process has its distal end truncated and provided
with a large oval opening (na, nf), which, as is evident from the de-
scription of Epipetalichthys below, must at least in part be the nasal
opening. Accordingly, the process contains the nasal cavity and rep-
resents the olfactory capsule. We thus have here an olfactory capsule
that, as in Elasmobranchs, is fairly independent and projects laterally
in relation to the other internasal parts of the ethmoidal region. (Cf.
Allis 1923, pp. 126-147.)
A closer examination of the nasal opening shows that this, by a
process of bone that projected forwards from the posterior wall, is
incompletely subdivided into a dorso-lateral (na) and a ventro-medial
130 Field Museum of Natural History — Geology, Vol. IV.
(»/) portion. Both of these portions are directed ventro-laterally, but
the latter considerably more so than the former, so that it in fact is
situated on the ventral side of the nasal capsule (Figs, i, 5;
PI. XXI ; PI. XXIII ; PI. XXIV, fig. 1). I was at first inclined to con-
sider them as corresponding to the two nasal apertures generally found
in fishes, and to think that the process that separates them would be
homologous with a part of the ala nasalis of Elasmobranchs (Allis
1923, pp. 126-147), and that the ala nasalis would be continuous with,
and form an integral part of, the nasal capsule. But after having read
the fine memoir on Chlamydoselachus recently published by Allis
(1923) I arrived at the conclusion that what has been called above the
ventromedial portion («/) of the nasal opening, really represents a lat-
eral part of the nasal fontanelle of Chlamydoselachus and certain other
Selachians and that, accordingly, the other portion (na) alone is the
nasal opening. The ala nasalis or its possible homologue, if there were
one, was probably entirely without ossification. If this opinion is true,
the vein commissure that in Selachians (Allis 1923, pp. 126-147),
Saurichthys (Stensio 1925), and probably in most fishes (Allen 1905,
PI. 1, fig. 1; PI. Ill, figs. 17, 18), connects the orbito-nasal vein and
the anterior facial vein with each other must, obviously, have traversed
the posterior part of the nasal fontanelle. Whether this fontanelle
was entirely occupied by a membrane of connective tissue or whether
it was more or less occupied by cartilage it is not possible to decide.
From what has been set forth regarding the nasal capsule, it is
clear that M. rapheidolahis in this regard much resembles the Elas-
mobranchs.
The cavum cerebrale cranii extends only a short distance into the
posterior part of the region, which, therefore, apart from certain
canals, in the whole of its interior consisted of cartilage. On account
of this fact we cannot conveniently consider it to be composed of a
number of walls, as in the case of the other regions, but will have to
deal with it as a solid body with five external surfaces which are well
bounded from each other. These surfaces are the following : a ventral,
a posterior, a dorsal, and a right and a left lateral.
The ventral surface is broad and large and is posteriorly continuous
with the ventral surface of the orbitotemporal region (Fig. 1 ;
Pis. XXI-XXIII). Unfortunately it is not fully preserved in its
anterior parts. As is seen from the figures (the transversal section
reproduced in Fig. 9 and PI. XXIV, fig. 1) it is slightly concave
both in transversal and rostro-caudal direction. And, as has been
pointed out above, it is covered by the external bone layer
Macropetalichthyids and other Arthrodires — Stensio 131
throughout its extension. Not far from each lateral border it has a
distinct sulcus {s.pal, Figs. 1, 9; PI. XXII), which leads forward
for some distance from the opening c.palz (Fig. 1; PI. XXII).
This sulcus was evidently developed for the r. palatinus facialis, and
may, perhaps, in addition, also have lodged a small arterial branch
given off from the arteria carotis interna through the foramen c.pal3.
So far as can be judged there is no indication of an articulation facet
for the palatoquadrate on the ventral surface of the region.
Close, medially, to each groove for the r. palatinus just described,
and about midway between the anterior end of the region and the
foramen c.palz, the external bone layer seems to show faint indications
of an ossification centre, a centre which, if it really exists, is so situ-
ated that the bone which it represents corresponds at least to the
exethmoid (lateral ethmoid) of fishes in general, and perhaps, in addi-
tion, to the preethmoid of certain fishes. (Cf. Allis 1897, PI. 21, figs.
8-10, SMX; 1898, pp. 446-450; 1909a, pp. 17-22; Stensio 1921, pp.
61, 93-94; I922, PP- 184-186; 1923, pp. 1247-1248; 1925, etc.) A
positive decision in this respect is, however, not possible until more
and better preserved material is available for investigation.
The posterior surface of the region, if we next turn to this, is,
by the anterior part of the orbitotemporal region above the orbital
floor, subdivided into a right and a left half, each forming the anterior
surface of the orbit of its side. As it could not be cleaned from
matrix, I can state little concerning it. It may have had a foramen
through which the r. ophthalmicus superficialis after traversing the
orbit entered the region, but nothing can be said about the posi-
tion of this foramen. As has already been pointed out above, there
probably was no anterior myodome developed, but a positive de-
cision in regard to this point is evidently not possible until material is
found which- will enable us to undertake a detailed examination of the
posterior parts of the region. The olfactory nerve did not traverse
the orbit and accordingly it had no foramen on the posterior surface
of the ethmoidal region.
The dorsal surface, like the ventral one, is large and broad and
posteriorly continuous with the dorsal surface of the orbitotemporal
region (Figs. 3, 5, 9; Pis. XIX-XX; PI. XXIV, fig. 1; PI. XXV,
figs. 1, 2; PI. XXVI, figs. 1, 2). It is slightly convex, both in trans-
versal and rostro-caudal direction (Fig. 9), and slopes slightly for-
ward. In its most anterior parts it is not preserved in the specimen
examined, being either crushed together with the dermal cranial roof
or destroyed in some other way. From the closely allied Epipetalichthys
132 Field Museum of Natural History — Geology, Vol. IV.
to be described below, we are, however, justified in concluding that the
conditions were about as shown in Fig. 3, i.e., that there probably
was a deep, broad, antero-dorsally directed depression (pfg) at the
anterior end of the dorsal surface, and that this depression was lined
throughout by a perichondral bone layer. A similar depression seems
among adult fishes to occur only in Elasmobranchs (Cf. Gegenbaur
1872, Pis. IV-VIII, etc.; Allis 1913; 1923, pp. 127-130; Fritsch
1895, figs. 228, 230; Woodward 1916-1919, PI. II, fig. 1, PI. XXVI,
fig. 3; Daniel 191 5, PI. I, fig. 1, etc.), and we would accordingly have
here a very remarkable agreement between Epipetalichthys and Macro-
petalichthys on the one hand, and the Elasmobranchs on the other. As
we shall see from the account given below, a depression of the same
sort probably occurred in all Arthrodires. In Elasmobranchs the de-
pression, which is described by Allis under the name of cavum pre-
cerebrale, is filled by a fatty tissue and this was probably the case
also in the Arthrodires, since the dermal bones in these do not bend
down so that they could have been directly superimposed on the bottom
of the depression, but are situated on a level with the upper borders
of this. This relatively loose position of the dermal skeleton in rela-
tion to the primordial neurocranium at the anterior end of the ethmoidal
region explains why the dermal skeleton is generally in a very im-
perfect state of preservation there. While in Elasmobranchs the cavum
precerebrale is separated from the cavum cerebrale in most cases merely
by membranous tissue, there was here in Macropetalichthys rapheido-
labis and in Epipetalichthys certainly a thick wall of cartilage between
them. In Lcemargus borealis I have also found that the wall between
the cavum precerebrale and the cavum cerebrale is to a large extent
formed by cartilage. This is also partly shown in White (1892, PI. I,
%. 5)-
From the dorsal side of the anterior part of the cavum cerebrale
cranii a rather wide canal goes dorsally and forward to the upper sur-
face of the region, where it opens with a large oval foramen (par,
Fig. 3; Pis. XIX-XX; PI. XXVI, fig. 1). This canal, as we shall
see, lodged the pineal or parietal organ or both these structures.
Apart from this canal, several fine ones for vessels and nerve
branches naturally opened on the dorsal surface of the region. In
Pis. XXI and XXIII we can see one of this sort (rm.ophth.lat), that
goes from the canal for the r. ophthalmicus lateralis (c.ophth.lat) a
short distance upwards and forwards to the anterior part of the supra-
orbital sensory canal (soc). This sensory canal being, like the others,
situated in a ridge on the lower side of the dermal bones, there must
Macropetalichthyids and other Arthrodires — Stensio 133
evidently have been a paired groove corresponding to the ridge, a
groove running as is shown by text fig. 3.
Finally, the lateral surface of each side (Figs. 5, 9; Pis. XXI-
XXIII; PI. XXIV, fig. 1; PI. XXV, figs. 1, 2) is strikingly long,
but not particularly high. It faces laterally and slightly downwards
and is somewhat concave in dorso-ventral direction. From its antero-
ventral part projects the large nasal capsule (olf) already described,
but, as is seen from text fig. 5 and PI. XXIV, fig. 1, this does not
occupy more than the ventral half of the height of the surface at this
place. As far as can be seen, there is no articulation facet for the
palatoquadrate, either on the surface itself or on the posterior or ventral
parts of the nasal capsule. This, together with the fact pointed out
above that there was no articulation facet on the ventral surface either,
seems to indicate that the palatoquadrate actually did not articulate
against the region, but was suspended beneath this with ligaments in
about the same manner as in the Selachians.
Ll =»„■-#*
g±?^.
a.pal
Fig. 9. Macropetalichthys rapheidolabis
Transverse section through the posterior half of the ethmoidal region, slightly
anterior to the anterior end of the cavum cerebrale cranii. Layers of substitution
bone denoted by strong black lines ; cartilage dotted ; dermal bones with vertical
striation. XK-
Li, Mi, dermal bones of the cranial roof (their boundaries not fully correct) ;
soc, supraorbital sensory canal ; s.pal, groove for the r. palatinus facialis and
perhaps for an arterial branch too; vx, proximal part of a canal running up-
wards in the cartilage, probably for some venous vessel; I, canalis tractus ol-
factorii.
In this connection it may also be pointed out that the postnasal
part of each lateral surface is very large, a condition probably caused
by the insertion of a powerfully developed part of the musculus con-
strictor superficialis (I) homologous to the muscle of the Selachians and
Teleostomes generally, known as the m. levator labii superioris (Vetter
1874, etc.), levator maxilla; superioris (Allis 1897, pp. 552-556), ab-
ductor B (Allis 1901, PI. II, fig. 4; PI. 12, fig. 5) or preorbitalis
(Luther 1909, pp. 36-39). If this were not the case, it would be
very difficult to find a satisfactory explanation for the considerable
post-nasal extension of the lateral surfaces.
134 Field Museum of Natural History — Geology, Vol. IV.
On the posterior part of each lateral surface there are found the
anterior openings of two fairly wide canals (c.ophth.sup.V ? and vy,
Fig. 5 ; PI. XXII ; PI. XXIII ; PI. XXIV, fig. i ) which evidently issue
from the orbit. But as the position of the posterior openings of these
canals and their course within the preorbital process are so far un-
known, it cannot be positively decided what their functions were. The
dorsal one {c.ophth.sup.V?) possibly transmitted the r. ophthalmicus
superficialis trigemini, the ventral one either vessels or the r. maxillaris
trigemini and the n. buccalis lateralis or both these nerves and vessels.
From the anterior end of the cavum cerebrale cranii, a paired,
rather low but wide canal (I, Figs. 9, 10, 11, 12, 13; Pis. XXIII,
XXVII) issues in a forward and somewhat lateral direction to the nasal
fossa of its side, being during the entire length situated very low in
the region. This canal, which narrows anteriorly, lodged, as far as I
can judge, the tractus olfactorius and, at the most anterior, probably
somewhat widened end, the lobus olfactorius. It may, therefore, be
called the canalis tractus olfactorii. That it would lodge a long olfactory
nerve and that the lobus olfactorius would have been situated at the
front end of the telencephalon is not, as will be set forth below, in
accordance with what we know concerning the features of the brain
otherwise. From its dorso-medial side there are given oft" in the
Field Museum specimen two canals of finer calibre which run antero-
dorso-laterally within the interior of the region {yx, vx\, PI. XXVI,
fig. 1), and which probably were traversed by veins draining the car-
tilage and dorsal surface.
Close below the groove for each supraorbital sensory canal, we
find the canal for the r. ophthalmicus lateralis (c.ophth.lat, Fig. 9;
Pis. XXI, XXIII), from which, as already pointed out, branches
(rm.ophth.lat) run upward to the supraorbital sensory canal.
Cavum Cerebrale Cranii and Brain
Although being partly much narrower than the primordial neuro-
cranium, the cavum cerebrale cranii (cv, Figs. 2, 4, 8, 10, 11 ; Pis. XX,
XXI, XXIII; PI. XXIV, fig. 3; PI. XXV figs. 1, 2; PI. XXVI, figs.
1, 2; PI. XXVII) has nevertheless a rather considerable size. Ob-
serving its shape, it may be considered to consist of three divisions,
which in the subsequent description are called the posterior, middle and
anterior divisions respectively.
The posterior division is limited entirely to the posterior narrow
division of the occipital region, and, in accordance with this, it is
much narrower and lower than the other two {pd, Figs. 10, 11; PI.
Macropetalichthyids and other Arthrodires — Stensio 135
XXVII). Its narrowest part is situated anteriorly, just at the tran-
sition to the middle division. Behind this place it grows slowly and
gradually. broader and a little lower backwards, but whether it did so
quite to the posterior end or not is unknown. Two transverse sections
through its posterior half somewhat anterior to the cranio-spinal
processes are seen in text fig. 2.
The middle division (md, Figs. 10, 11) extends through the
anterior division of the occipital region, the whole labyrinth region and
a short posterior part of the orbitotemporal region (Cf. also Figs.
2, 4, 8; Pis. XX, XXIII; PL XXIV, fig. 3; PL XXV, figs. 1, 2; PL
XXVI, figs. 1, 2), and is the largest one of the three with regard to
height and breadth, but its length is about equal to that of the pos-
terior division. Its maximum height is situated approximately at the
transition between the occipital and labyrinth regions, decreasing from
there both forward and backward, although only very slightly for-
ward. The bulge caused by the vagus and the other nerves, not taking
into consideration the width, attains its maximum in the front end of
the division, in the most anterior part of the orbitotemporal region,
from which it narrows slightly both backward and forward.
The anterior division (ad, Figs. 10, 11) extends through the an-
terior larger part of the orbitotemporal region and a very short
posterior part of the ethmoidal region, where, at the very anterior end,
it divides into a right and a left half, each of which anteriorly is con-
tinued by the canalis tractus olfactorii (Figs. 10, 11; PL XXIII;
PL XXVI, figs. 1, 2). It is well bounded from the middle division,
being both lower and narrower than the anterior part of this. The
height gradually diminishes forward throughout the length of the divi-
sion, finally becoming equal to that of the posterior end of the canalis
tractus olfactorii, while the breadth, on the contrary, increases slightly
in the same direction. The length is only about half as great as in
the other two divisions.
The floor of the middle division is slightly convex, that of the
posterior division slightly concave, and so is also that of the anterior
division, although the concavity is somewhat larger there (Figs.
10, 13; PL XXVI, figs. 1, 2) and forms a shallow, indistinct fossa.
In the median parts, this fossa has, in its turn, a small, very slightly
indicated depression (fhy, Fig. 10), which is so situated that it
must have lodged at least the most ventral part of the hypophysis.
The lobi inferiores probably had their position laterally of this de-
pression, which may represent the last remnant of an originally more
pronounced and large fossa hypophyseos.
136 Field Museum of Natural History — Geology, Vol. IV.
,.JII ..c.opkth.SupV
VII
Fig. 10. Macropetalichthys rapheidolabis
Shape of the cavum cerebrale cranii in ventral view. With the exception
of the posterior division, which is restored from specimens in the American
Museum, entirely after the Field Museum specimen. X24.
ad, anterior division of the cavum cerebrale cranii; afr, acustico-facialis
recess ; a.opt, division of the external opening of the opticus canal, probably for
the arteria optica; c.bucc.lat, canal for the lateralis fibres, representing the n.
buccalis lateralis and possibly also for certain general cutaneous fibres to the
n. facialis ; c.car.int, canal for the arteria carotis interna ; c.ophth.lat, canal for
the n. ophthalmicus lateralis ; c.ophth.prof, canal for the r. ophthalmicus pro-
fundus; c.ophth.sup.V , canal for the r. ophthalmicus superficialis trigemini; c.pah,
canal for the r. palatinus facialis from the facialis canal to the orbit; dx, canal
for a dorsal branch from the n. linese lateralis ; fhy, depression probably for the
Macropetalichthyids and other Arthrodires — Stensio 137
hypophysis; md, middle division of the cavum cerebrale; n.l, canal for the
n. lineae lateralis ; pd, posterior division of the cavum cerebrale cranii ; v.lb, canal
leading from the labyrinth cavity to the vagus canal, probably for a large vein;
I, canalis tractus olfactorii ; II, division of the opticus canal for the n. opticus ;
III, canal for the n. trochlearis; V, proximal wide part of the trigeminus canal,
forming a trigeminus recess ; V»,», distal narrow part of the trigeminus canal
traversed by the r. maxillaris, r. mandibularis trigemini and the n. buccalis
lateralis ; VII, facialis canal ; VIII, acusticus canal ; IX, glossopharyngeus canal ;
Xm, vagus canal transmitting the n. vagus and the v. cerebralis posterior.
In the posterior part of the roof of the anterior division, there is,
in the Field Museum specimen, a median, unpaired diverticle (ep)
situated as shown by Fig. 11, i.e., it extends a short distance
upward into the dorsal wall of the orbitotemporal region. Somewhat
in front of this diverticle, the canal par to which reference has already
been made issues from the same division in an antero-dorsal
direction to the dorsal surface of the posterior part of the ethmoidal
region, opening there with a rather large, oval foramen (par, Figs.
3, 11-13; Pis. XIX, XX; PI. XXVI, fig. 1).
According to the observations recently made by Woodward (1922,
p. 31, fig. 3) the pit that regularly occurs on the lower side of the
pineal plate in non-Macropetalichthyid Arthrodires is paired in Tita-
nichthys, a fact which causes Woodward to consider that the epiphysis
there was a paired, symmetrically situated organ. We are thus in
Titanichthys concerned with a form in which the parietal and pineal
organs probably were situated beside one another (Cf. Goodrich 1909,
p. 25 ; Plate 1923, pp. 743-753) and it seems to be a matter of course
that in all typical non-Macropetalichthyid Arthrodires, as, for instance,
Dinichthys and Coccosteus, they must have been situated exactly as
in Titanichthys.
In Macropetalichthys rapheidolabis the bone-plate Mx (text fig. 15)
of the dermal cranial roof with its posterior part evidently corresponds
to the pineal plate (Pi, Fig. 14) of Titanichthys and other typical
non-Macropetalichthyid Arthrodires. The canal by which it is
said by Eastman (1908a, p. 104; 1908b, p. 169) to be perforated
seems to have such a position that it must form the distal continua-
tion of the canal par, and at least ventrally correspond to the pit on
the lower side of the pineal plate in the non-Macropetalichthyid Arthro-
dires. These facts, together with the circumstance that Macropetalich-
thys rapheidolabis in most other respects is fairly closely allied to the
other Arthrodires, make it highly probable that the parietal and pineal
organs in it were situated as in these. If, therefore, these organs in
the other Arthrodires occupied a position beside one another, they
would obviously in M. rapheidolabis both have been situated in the
canal par, and the diverticle ep would thus have lodged no part of the
138 Field Museum of Natural History — Geology, Vol. IV.
brain and might in such a case very well be thought to be occasionally
developed in the Field Museum specimen. That the diverticle ep can-
not probably have lodged the pineal organ is also, as kindly pointed
out to me by Professor N. Holmgren of Stockholm, indicated by its
position very far back.
Leaving this question, we turn to the canals that issue from the
three divisions of the cavum cerebrale cranii, beginning posteriorly and
proceeding forward.
It would be expected that several canals for spino-occipital nerves
had left the posterior division of the cavum cerebrale cranii, but,
as pointed out above, no such canals could be observed in the
material so far investigated, a fact which of course does not neces-
sarily imply that they are lacking, for the possibility is to be taken
into consideration that they are very minute and on that account can
only exceptionally be distinctly seen, and then only in especially well-
preserved specimens.
From the middle division of the cavum cerebrale issues most pos-
teriorly the wide vagus canal (Xvn Figs. 10, 11, PI. XX; PI. XXIII;
PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2). Some distance anterior of
this appears the glossopharyngeus canal (IX) and most anteriorly,
rather close to each other, the acusticus canal (VIII), the facialis canal
(VII) and the trigeminus canal- (V). The acusticus and facialis canals
take their origin from a common recess on the side of the division, a
recess which I have termed the acustico-facialis recess (afr). All the
canals enumerated leave the basal parts of the division. Besides these
there are, however, three other canals situated higher up, all of which
must have been traversed by vessels. The most posterior one of these,
(v.lb, Fig. 11; PI. XX; PI. XXV, figs. 1, 2; PI. XXVI, figs. 1, 2),
which is very wide and goes from the postero-dorso-medial parts of
the labyrinth cavity to the dorsal part of the vagus canal at the tran-
sition between this canal and the division in question of the cavum
cerebrale, probably transmitted a vein that drained the labyrinth. The
two remaining canals (vlt v2, Fig. 11; PI. XX; PI. XXV, figs.
1, 2; PI. XXVI, figs. 1, 2, 3) are both narrow, connect the dorsal parts
of the division above the acustico-facialis recess with the labyrinth
cavity and most probably may have been traversed by arteries.
From the anterior division of the cavum cerebrale cranii, the oculo-
motorius canal (III, Fig. 11; PI. XXIII; PI. XXV, figs. 1, 2; PI.
XXVI, figs. 1, 2), the opticus canal (II) and the canal for the tractus
olfactorius (I) take their origin, the one last mentioned from the an-
terior end and the one first mentioned far posteriorly, not so very far
Macropetalichthyids and other Arthrodires — Stensio 139
from the middle division. Besides these canals there issue from the
anterior division, as we have already seen, the canal par and the diver-
ticle ep.
Guided by the detailed knowledge we possess of the canals for the
cranial nerves, we are to a certain extent able to draw conclusions con-
cerning the brain and its position in the cavum cerebrale. From the
place of exit and direction of the acusticus facialis and trigeminus
canals, it is obvious that the roots of the n. acusticus, n. facialis and
n. trigeminus must have left the medulla oblongata in the anterior half
of the middle division of the cavum cerebrale and, accordingly, that
the medulla oblongata reached far forward in the middle division (myl.
Figs. 12, 13). The most anterior part of this division was, how-
ever, certainly occupied by the whole, or, at least, the larger posterior
portion of the mesencephalon (tnes, Figs. 12, 13). Dorsally of the
mesencephalon and the anterior portion of the medulla, there probably
c.ophth.Jat par
c.ophih.sup.V
op \
Fig. 11. Macropetalichthys rapheidolabis
Shape of the cavuum cerebrale cranii in lateral view. After the Field Museum
specimen, with the exception of the posterior division, which is restored after a
specimen in the American Museum. X^-
ep, canal of doubtful importance, probably an occasional structure ; par, pineal
canal, probably for both the parietal and pineal organs (at least in the basal half) ;
'1. v», canals probably for arteries from the cavum cerebrale cranii to the
labyrinth cavity ; vx, vx,, canals probably for veins. Other letters of reference
as in text fig. 10.
existed a rather well-developed cerebellum (met, Figs. 12, 13) and
the anterior half of the middle division may accordingly have been
rather well filled, while the posterior half of the same division, which
lodged only the posterior longer portion of the medulla oblongata,
cannot have been filled by this. The conditions there must have been
about as seen in Figs. 12, 13. The medulla was undoubtedly long,
but it is impossible to say with certainty how far back it extended.
The anterior division of the cavum cerebrale must, as is easily
understood, have lodged the telencephalon and diencephalon (Figs.
12, 13), and in addition probably a short anterior portion of the mesen-
140 Field Museum of Natural History — Geology, Vol. IV.
cephalon. The diencephalon (die) seems to have been long, probably
reaching forward to about the middle of the division, but, may as far as
can be judged, not have been so high as this. The hypophysis, and, if
they were developed, the saccus vasculosus and the lobi inferiores as
well, may have been situated in the shallow concavity in the floor of
the anterior division approximately in the way shown in text fig. 13.
The hypophysis occupied, however, a small depression of its own in the
middle part of the concavity (fhy, Fig. 10). The canal par, prob-
ably lodged both the parietal and pineal organs, while the importance
of the diverticle ep is still doubtful.
The telencephalon (tel, Figs. 12, 13) probably occupied about
the anterior half of the anterior division of the cavum cerebrale. It
must have been comparatively broad and low and anteriorly it was
probably bilobated, a portion of it on each side extending into the
proximal part of the canalis tractus olfactorii of its side. From each
of the two anterior lobes the tractus olfactorius (I) went through its
long canal anteriorly and laterally to the lobus olfactorius (l.olf),
which, as far as can be judged, was situated close to the nasal pit.
From what has now been said it will be evident that the cavum
cerebrale cranii only to a certain extent reflects the shape of the brain.
But despite this it seems to be rather clear that the brain cannot have
resembled that in Dipnoi, Crossopterygii or Actinopterygii, but that it
may have been Elasmobranchian-like. At least with regard to certain
anterior parts it probably was much as in Hcptanchus (Cf. Gegenbaur
1898, Fig. 455) and Chalmydoselachus (Allis 1923, PI. 22, fig. 59).
The restoration of the brain given in text figs. 12 and 13 is based
chiefly on the conditions in Heptanchus and is of course to a large extent
rather arbitrary.
DERMAL BONES OF THE PRIMORDIAL NEUROCRANIUM
The ventral side of the primordial neurocranium was entirely with-
out dermal bones, the parasphenoid of Cope's (1891, p. 453, PI. 29,
fig. 2; PI. 30, fig. 1) and Eastman's (1908a, pp. 108-109; 1908b, pp.
174-175) descriptions being simply the external bone layer on the
ventral side of the primordial neurocranium.
The dorsal side of the primordial neurocranium, on "the contrary,
is, as known, completely covered by a cuirass of dermal bones, a cuirass,
which, in this paper, is generally referred to as the dermal cranial roof.
On account of the postero-medial displacement of the eyes, the orbital
openings have become entirely encircled by the dermal cranial roof.
Macropetalichthyids and other Arthrodires — Stensio 141
Fig 12. Macropetalichthys rapheidolabis
Hypothetical shape of brain from above in relation to the cavum cerebrale
cranii. Brain with a dark tone, outlines of the cavum cerebrale indicated by a
continuous line. X?4-
die, diencephalon ; l.olf, lobus olfactorius; mes, mesencephalon; met, cerebel-
lum ; myl, medulla oblongata ; nac, nasal pit ; par, parietal or pineal organs or
both; tel, telencephalon; I, tractus olfactorius; II, n. opticus; III, n. oculo-
motorius; V, roots of the trigeminus proper; VII, facialis roots + lateralis
roots of the entire trigemino-facialis complex; VIII, root of the n. acusticus ;
IX, root of the n. glossopharyngeus ; X, roots of the n. vagus and the n. lineae
lateralis.
142 Field Museum of Natural History — Geology, Vol. IV.
The general shape of the dermal cranial roof may be seen in
Fig. 15. The restoration given of it by Dean (1901, Fig. 12, p. 116)
is almost correct, while the one published six years later by Eastman
is wrong with regard to its general proportions (Eastman 1908a,
Fig. 19), as becomes evident by comparing Eastman's restoration with
the specimen he figured in PI. rrof his description.
Concerning the number of bones composing the dermal cranial roof,
1 have nothing new to add here to what is known from earlier descrip-
tions. The boundaries between the various bones, on the other hand,
I have in certain places found to be somewhat different from those
shown in Dean's (1901) and Eastman's (1908a) restorations. But
my own restoration (Fig. 15) does not claim to be fully accurate
in this respect, as the material investigated by me was both very
limited and imperfectly preserved with regard to the dermal cranial
roof.
Fig. 13. Macropetalichthys rapheidolabis
Hypothetical shape of brain from the left side in relation to the shape of the
cavum cerebrale cranii. Brain with a dark tone, outlines of the cavum cerebrale
indicated by a continuous line. X24-
hyp) hypophysis and other ventral parts of the diencephalon. Other letters
of reference as in text fig. 12.
As in a paper which I expect to publish later, I think I can show
that the dermal bones of the cranial roof in Arthrodires have evolved
independently of those in the Telcostomi, or, to put it differently, that
they are not directly comparable to those in the Teleostomi, I have let-
tered them here in text fig. 15 with indifferent letters.
Each plate 5" has, posteriorly, close to the posterior border, a ver-
tically descending lamina which is rigidly fixed to the cranio-spinal
process of its side. In the specimens investigated by me, this lamina
is unfortunately very imperfectly preserved, so that its extension and
relations could not be established with certainty. In M. pelmensis it
is, however, well-preserved, and it seems there to cover the posterior
surface of the cranio-spinal process and to be very large and meet its
fellow of the opposite side in the median line.
Macropetalichthyids and other Arthrodires — Stensio 143
Hennig in his description of M. pehncnsis, did not perceive the
true nature of this lamella, but considered it to be a paired lateral
occipital (Hennig 1907, pp. 585-586, Fig. 3, OL)1. This ventral lamella
is necessary for the support of the much lengthened, posterior parts of
the dermal cranial roof. A parallel to this is found in sturgeons
(both Polyodontids and Acipenserids) in which a lamella issues from
each extrascapular bone downwards in such a way that it covers and
is fixed to the anterior surface of the cranio-spinal process of its side.
Accordingly, it is not homologous with that in Macro petalichthys.
In Saurichthys, in which also a strong cranio-spinal process, very sim-
ilar to that in sturgeons, occurs, a descending lamina from the dermal
cranial roof is, on the contrary, entirely lacking (Stensio 1925).
The plate M is said by Eastman (1908a, p. 104; 1908b, p. 169)
to be pierced by a fine canal just above the canal par in the roof of the
primordial neurocranium. The material investigated by me was not
satisfactorily preserved at this place, and I have therefore not been
able to verify the correctness of Eastman's statement as to this point.
Fig. 14. TlTANICHTHYS AGASSIZI, NEWB.
Dermal cranial roof and infraorbital plate. After Eastman (1008a. fig. 29).
Sensory canal grooves omitted. Much diminished. Pi, pineal plate; R, rostral
plate; orb, orbital opening.
DERMAL BONES OF THE CHEEK
No remains of dermal bones were found which, with certainty, be-
longed to the cheek, and it is rather probable that these bones were
either much reduced or entirely lacking in the fish. That the plate Plf
situated latero-ventrally of the orbital opening, is no infraorbital bone,
is evident both from the fact that the eye has shifted a rather consider-
able distance postero-medially and from the course of the sensory canals,
as we shall understand from the account of these given below.
'The description which Hennig (1007) gives of M. pelmensis is also in most
other respects very incorrect, the anatomical features of the fish having been
for the most part entirely misunderstood.
144 Field Museum of Natural History — Geology, Vol. IV.
VISCERAL SKELETON
Remains of the visceral skeleton have hitherto not been found.
Despite this we are, however, able to draw certain conclusions about it.
The palatoquadrate was not coalesced with the primordial neuro-
cranium, but was an independent element. As far as we can judge,
it did not articulate with the primordial neurocranium, either anteriorly
or posteriorly, and, if this be true, it must have been suspended merely
by ligaments below the ethmoidal region and by means of the hyoid arch
below the labyrinth region. Accordingly, its relations to the primordial
neurocranium seem to have been about as in sturgeons and recent sharks
(except the Notidanide, in which, as known, there exists a postorbital
articulation between the palatoquadrate and primordial neurocranium).
cpp
Fig. 15. Macropetalichthys rapheidolabis
Restoration of the dermal cranial roof. Course of the sutures probably in
places not fully correct. Sensory canals marked with double dotted lines and
colors. Ornament of the bones omitted. XM-
Li, L2, Ls, Mi, M2, Pi, P2, S, dermal bones of the cranial roof ; c.pp, sensory
canal commissure, probably corresponding to the posterior head lines of pit origin
in recent fishes (red) ; ifc, infraorbital sensory canal (blue) ; Ic, cephalic divi-
sion of the lateral line( red) ; olf, nasal capsule; orb, orbital opening; poc, pre-
opercular sensory canal (yellow) ; soc, supraorbital sensory canal (green).
Macropetalichthyids and other Arthrodires — Stensio 145
Probably there was in relation to the palatoquadrate, a large musculus
preorbitalis (Cf. the description of the ethmoidal region above).
If my interpretation of the canal c.hy as the canal for the vena
hyoidea is correct, it is obvious that the hyoid arch articulated against
the primordial neurocranium closely anterior to the external opening
of this canal, or, at least that the hyoid arch, if it did not articulate
against the primordial neurocranium, had its dorsal end situated closely
anterior to this place, for it must of course have been situated an-
terior to the hyoid vein. Or to express it in another way, the hyoid
vein must have passed upward to the jugular vein along the pos-
terior side of the hyoid arch. Whether the dorsal end of the hyoid
arch was situated ventrally of the vena jugularis, as in Selachians, or
dorsally of the same vein as in the Teleostomi, it is impossible to decide
from the specimens investigated (Cf. Allis 1915, 1918b).
Of much interest in this connection is the fact that the jugular
vein as well at the upper end of, as anterior and posterior to the hyoid
arch, went within the lateral wall of the primordial neurocranium and
that, accordingly, a shifting of the place of articulation of the dorsal
end of the hyoid arch against the primordial neurocranium could take
place in a dorsal or ventral direction without affecting the jugular vein.
As the jugular vein in Eusthenopteron1 also passed within the lateral
wall of the primordial neurocranium to about the same extent as in
Macropetalichthys and probably did also in other Crossopterygians
(Cf. Stensio 1922, p. 201, the long intramural course of the truncus
hyoidemandibularis facialis) one suspects that this was originally the
case in all fishes.
If this suspicion be true, the mainly extramural course of the jugu-
lar vein in most fishes would be due to reduction of certain lateral
parts of the lateral walls of the primordial neurocranium. And, more-
over, it would be very easy to understand how the place of articulation
of the hyoid arch could have shifted upwards to the dorsal side of
the jugular vein in the Teleostomi. Finally, the hyomandibular of the
Teleostomi, would not, as Allis (1915, 1918b) supposes, have incorpo-
rated at its dorsal end certain parts lacking as such in the hyomandib-
ular of the Elasmobranchs. The conditions found in Macropetalich-
thys and Eusthenopteron with regard to the vena jugularis and the vena
hyoidea thus seem to make certain of the views recently advanced by
Allis (1915, 1918b) concerning the homologies of the hyomandibular
of fishes very doubtful.
'Observation made on a well preserved specimen in the collection of the
Palseontological Institution of Upsala.
146 Field Museum of Natural History — Geology, Vol. IV.
The first branchial arch followed not very far behind the hyoid
arch. On account of the cranio-vertebral articulation found in most
or all Arthrodires, there is no reason to believe that the branchial basket
extended backwards into the abdominal region of the trunk, but was
limited to the suboccipital part q£ the head.
DENTITION
Unfortunately the dentition is so far entirely unknown.
SENSORY CANALS OF THE HEAD
The sensory canals are found preserved on the head only in the
dermal cranial roof. They consist there of tubes which are situated
within the bones, or, more exactly, in ridge-like thickenings on the
lower side of the bones (Figs. 4, 8, 9). Accordingly they have a
very deep position (Cf. Kaer 1915, p. 12), a fact which explains that
there are frequently traces of them preserved even in specimens in
which the dermal bones have almost entirely weathered away, as, for
instance, in the specimen reproduced in Pis. XIX, XX. At least in
the larger parts of their extension they open outwards with a double
series of fine tubuli. These tubuli occur in great number and are
situated fairly close to each other, conditions which seem to indicate
that they were much more numerous than the sensory canal organs.
In any case they are much more numerous than the nerve-canals lead-
ing to the sensory canals. ,
On account of the fortunate circumstance that the fine canals for
the nerve branches to the sensory canal organs are lined by a thin
layer of bone, they could be preserved, and thus enable us to get a
rather complete knowledge of the manner in which the sensory canals
were innervated. With the help of this knowledge it is not difficult
to homologize the sensory canals in Macropetalichthys rapheidolabis
with those in recent fishes, as will be evident from the subsequent ac-
count. With regard to the terminology, I shall follow Allis (1889;
1900; 1901 ; 1903; 1905; 1923) with the exception only that in accord-
ance with Ewart (1892), Ewart and Cole (1895), Ewart and
Mitchell (1892), and Cole (1896a, b; 1898) I limit the infraorbital
sensory canal to those parts innervated by the n. buccalis lateralis and
n. oticus lateralis.
The supraorbital sensory canal (soc. Figs. 8, 9, 15; Pis. XIX,
XX, XXI, XXIII) has in its present state of preservation its an-
terior end at the anterior border of the bone-plate Llt but in the living
fish it continued forward into the soft tissue anteriorly and laterally
to this plate. How it may have passed in this soft tissue it is of course
Macropetalichthyids and other Arthrodires — Stensio 147
not possible to say positively, but it seems to me most probable that
it was there as in Coccostens, and forms allied to this (Cf. Jaekel 1902
fig. to page 107; 1906a, text figs. 2, 3), i.e., that it very soon turned
backwards and went dorsally and behind the nasal opening down to
the infraorbital canal. It would thus, if I am right, have had the
same course in its anterior parts as in Elasmobranchs. (Cf. Garman
1888; Allis 1901, pp. 104-107, 1 18-128; 1905, p. 419, pp. 470-478;
1923; Ewart 1892, pp. 66-74; Hawkes 1906, pp. 960-961, PI. 68; cf.
also Cole 1896 a, b.) Within the bone-plate Lx it runs first backward
and slightly laterally to about the center of the ossification of the plate,
then fairly suddenly curves and takes a backward and slightly medial
direction. With this direction it continues backward to the plate M2,
which it traverses to about the center of ossification, and here joins its
fellow of the opposite side and a cross comissural sensory canal between
the cephalic division of the lateral lines in the way shown in text fig.
15. No anastomosis is found between it and the posterior part of the
infraorbital canal of its side.
To the supraorbital sensory canal go several fine branches from
the canal for the n. ophthalmicus lateralis. The number of these
branches could not, however, be established. The posterior ones issue
dorso-postero-medially, the anterior ones antero-dorso-laterally. In
Pis. XXI and XXIII one anterior branch (rm.ophth.lat) is clearly
seen issuing from the canal for the n. ophthalmicus lateralis (c.ophth.
lat) to the supraorbital sensory canal (soc).
Its considerable extension backwards and its direction in the pos-
terior parts, indicate with certainty that the supraorbital sensory canal
comprises not only the supraorbital sensory canal as found, for in-
stance, in Amia and many other fishes, but also the homologue of the
anterior head line of pit organs of these fishes. (Cf. Allis 1889, pp.
505-506; 1900, pp. 445; 1903, p. 187; 1905; Herrick 1901, pp. 222-
223; Stensio 1921, pp. 218, 263; 1922, p. 192; 1923, p. 1258; 1925.)
The infraorbital sensory canal (ifc, Figs. 8, 15; Pis. XIX, XX,
XXII; PI. XXIV, fig. 1) has its posterior end situated at about
the ossification center of the plate P2; being there continuous with the
cephalic division of the lateral line of its side. From this place it runs
forward and slightly medially into the plate Plt retaining the same di-
rection as behind this and being during this part of its course situated
latero-ventrally of the orbital opening. It does not, however, traverse
the bone-plate Px, in the entire length, but goes to the lateral border
of this some distance behind the anterior end as is shown in text fig.
15. After leaving the cranial roof at this point, it must have passed
148 Field Museum of Natural History — Geology, Vol. IV.
forward and downward on the cheek, probably through soft tissue, and
it is likely that it extended as far forward as to the rostrum, and, that
it, as pointed out, anastomized with the supraorbital canal behind the
nasal opening.
To the part of the infraorbital canal situated within the bone-
plates P1 and P2 lead a few fine canals which pierce the lateral parts
of the postorbital process and which very probably transmitted lateralis
branches from a nerve corresponding to the r. oticus lateralis of recent
fishes. Two of these branches (in) are partly seen in Pis. XX, XXII,
and it is easy to understand from their direction that they in any case
must have been traversed by lateralis branches that were given off from
a prootic nerve, i.e., a nerve that had its exit through the cranial wall
anterior to the labyrinth.
Before leaving the infraorbital canal, it is finally necessary to point
out that its course in the dermal cranial roof is incorrect in the restora-
tion published by Dean (1901, text fig. 12).
The cephalic division of the lateral line (Ic, text figs. 4, 15; Pis.
XIX, XX, XXIV) enters the dermal cranial roof at the posterior
border of the bone-plate S, traverses this bone-plate in anterior and
slightly medial direction, and continues with the same direction to the
ossification center of the bone-plate L3. There it suddenly takes an
antero-lateral course and goes with this to the ossification center of the
bone-plate P2, joining there, as already mentioned, the infraorbital canal.
At the place in the bone-plate L,, where the cephalic division of
the lateral line turns laterally, it gives off an antero-medial branch
which goes to the ossification center of the plate M2, joining there with
its fellow of the opposite side and forming in this way an angulated
transverse commissure (c.pp, text figs. 4, 15; Pis. XIX, XX; PI.
XXIV, fig. 3). At the angle in the median line this commissure
anastomozes with the posterior end of the supraorbital sensory canals
just after these have joined one another.
A short anterior portion of the cephalic division of the lateral line
just behind the infraorbital sensory canal was probably innervated by
the presumed lateralis branch that traversed the cranial wall, together
with the n. glossopharyngeus. The remaining part of the division
anterior of the transverse commissure, as well as the adjacent half
of this commissure, was supplied by a nerve that issued from
the n. linese lateralis already within the vagus canal and from there
went upward through the canal dx, (PL XXIV, fig. 2), and its
branches dxa and dxb (PL XXIV, fig. 2), thus by a ramus supra-
temporalis from the n. lineae lateralis. Finally, the part of the division
Macropetalichthyids and other Arthrodires — Stensio 149
behind the transverse commissure was innervated by a number of fine
branches which independently of each other issued from the n. lineae
lateralis, each of them probably supplying only a single sense organ.
In the anterior division of the occipital region where these nerve
branches pierced the primordial neurocranium, six of these canals are
seen in the Field Museum specimen (Id^ld* Pis. XIX, XX ; PI. XXIV,
fig- 3).
In the description of the anterior division of the occipital region it
was pointed out that a fine branch from the n. lineae lateralis prob-
ably went up to the dorsal surface of the primordial neuro-
cranium through the canal cl (text fig. 3; Pis. XIX, XX). This fine
lateralis branch thus had, if it really existed, such a course that it
must have innervated one or a few sensory canal organs situated
medially of the cephalic division of the lateral line posterior to the
cross commissural canal c.pp, but, as it is not a closed sensory canal
there, the presumed sensory canal organs supplied by it would have
formed a short, transverse pit-line. Unfortunately the material inves-
tigated did not enable me to decide whether there was a groove for
such a pit-line on the dorsal side of the dermal bones. The possible
non-existence of the groove need not, however, indicate absolutely
the absence of the pit-line, for this may in such a case very well be
thought to have been situated entirely in the skin and thus to have
been wholly unrelated to the underlying dermal bones.
If the pit-line in question really could be shown to exist, it would,
according to its position and innervation, evidently be homologous with
the supratemporal commissure in recent fishes, while the cross com-
missural canal c.pp would represent the posterior head line of pit organs
in these, a view which is also supported by the position and direction
of each half of the latter canal. (Cf. Allis 1889, PI. 42 ; 1901, pp. 101-
103, 116-117, PI. 10; 1903, pp. 186-187, PI. Ill; 1905, pp. 471, 474;
Ewart 1892, PI. II, fig. 2; Stensio 1923, p. 1258, fig. 4.) Below,
in the description of Epipetalichthys, we shall find additional facts
which point in the same direction.
According to the restoration published by Dean (1901, text fig.
12) a canal, which, in its position, corresponds very well to the
preopercular canal, issues in latero-ventral and slightly posterior di-
rection from the point where the infraorbital canal joins the cephalic
division of the lateral line. This canal is not found in Eastman's
restorations in 1908 (Eastman 1908a, text fig. 19; 1908b, text fig. 24),
and the material investigated by me was not preserved sufficiently well
to show with certainty whether Dean or Eastman is right in this
150 Field Museum of Natural History — Geology, Vol. IV. ,
respect. As the canal in question is found in a specimen described as
Macro pctalichthys agassizif below, and also occurs in most other
Arthrodires, it seems very likely that it is not missing in Macropeta-
lichthys rapheidolabis. On account of this it was drawn in text fig. 15,
in which it is designated by the letters foe. If. it really was present, it
must have continued down to -the cheek, where it chiefly, or, perhaps,
even entirely, had its passage through soft tissue.
REMARKS '
On account of the flat, broad shape of the primordial neurocranium
as a whole and the position and direction of the orbital openings, there
can be no doubt that M. rapheidolabis, like other species of Macropeta-
lichthys must have been a decided benthonic form.
The description given here makes it fully evident that many of the
strange anatomical features which Eastman thought he had observed
in M. rapheidolabis are mistakes.
MACROPETALICHTHYS AGASSIZI? (H. v. Meyer)
(PI. XXIX, figs, i, 2)
Through the courtesy of Mr. S. Junkermann of Bielefeld, Ger-
many, I had the opportunity of examining an imperfectly preserved
neurocranium of a Macropetalicthyid, which, as far as it is preserved,
agrees so well with M. rapheidolabis, that it at least provisionally seems
correct to refer it to the genus Macro pctalichthys. Among the species
described so far of this genus it seems to resemble most closely M.
agassizi, and on this account I have, with some hesitation given it this
specific name.
The primordial neurocranium is imperfectly preserved, being rep-
resented only by certain parts of the occipital region (PI. XXIX, fig.
1), which are much fractured and crushed. As far as can be seen,
it seems to have been less ossified than that of M. rapheidolabis, but
at least in the posterior, narrow division of the occipital region, I found
both the internal and external bone layers. Further forward the in-
ternal bone layer appears to have been entirely lacking. With regard
to its general shape, the primordial neurocranium seems to have con-
siderably resembled that in M. rapheidolabis.
In text fig. 16 we see two transversal sections through the most
anterior part of the posterior narrow division of the occipital region.
As is evident from these sections, the haemal groove did not extend so
far forward as in M. rapheidolabis, and it is further worthy of notice,
that, contrary to what is the case in that species, there was no bone
Macropetalichthyids and other Arthrodires — Stensio 151
tube around the notochord. In the anterior one of the two sections
figured, the postero-lateral-dorsal corners of the anterior broad division
of the region are seen in section, as they are extended considerably in
postero-lateral direction (pr, text fig. 16a). Of the dermal cranial
roof we find the plates M2, L3, P% and 5" (text fig. 16a; PI. XXIX,
fig. 2) more or less well-preserved. Their ornaments consist of small,
numerous, rather densely placed tubercles, which are stellate at their
bases.
The sensory canals are closed tubes situated deeply in the dermal
bones, as in M. rapheidolabis. They open outward with numerous,
straight, unbranched tubuli, but while these tubuli in M. rapheidolabis
are arranged in a double series, they form here only a single one from
each canal (PI. XXIX, fig. 2). It is quite probable that the canals for
,cr.od
«— er.ol
Fig. 16. Macropetalichthys agassizi?
Transverse section through the most anterior part of the posterior narrow
division of the occipital region. The section A taken only a short distance an-
terior to the section B. Cartilage dotted, layers of substitution bone with strong
black lines, dermal bones of the cranial roof with vertical striation. X'/s
cr.od, crista occipitalis dorsalis ; cr.ol, crista occipitalis lateralis ; ch, probable
position of the space for the notochord ; cv, cavum cerebrale cranii ; Ic, cephalic
division of the lateral line; pr, postero-laterally projecting corner of the anterior
broad division of the occipital region.
nerves leading from below to the sensory canals were much fewer in
number than the tubuli, a fact which enables us to conclude with a
rather great degree of probability, that the sensory canal organs did
not correspond in number to the tubuli but were much less numerous
than these.
152 Field Museum of Natural History — Geology, Vol. IV.
MACROPETALICHTHYS PELMENSIS, Hennig
The description given by Hennig (1907) of this species is, as I
found by examination of his type specimen, in several respects very
incorrect.
What Hennig (1907, pp. 585-586) takes to be a paired lateral
occipital seems in fact to be a ventral, paired process from the bone-
plate S, of the dermal cranial roof, according to the terminology em-
ployed by me in the present paper. This process, which meets its
fellow of the opposite side in a median suture and extends very low
downwards, covers the posterior surface of the probably very large
cranio-spinal process of its side.
The structure of the dermal cranial roof was also in other respects
very much misinterpreted by Hennig, a circumstance which is, how-
ever, so obvious that it will not be necessary to deal with it here.
Eastman's description of the species in 1908 (1908b, pp. 176-177)
is, with regard to the posterior parts of the neurocranium, also very
erroneous, as Eastman on that occasion did not even correctly recog-
nize the main divisions of the neurocranium, a fact which lead him to
certain very strange conclusions about the position of the ductus endo-
lymphaticus and the relations between this and the sensory canal sys-
tem (Cf. Kaer 1915, pp. 11-12).
It may finally be emphasized in this connection that the primordial
neurocranium probably is preserved in the specimen, and that a further
preparation of it would therefore be very desirable.
EPIPETALICHTHYS WILDUNGENSIS, Jaekel, gen. nov., sp. nov.
PI. XXVIII, figs. 1, 2; PI. XXIX, figs. 3, 4; PI. XXX, fig. 1;
PI. XXXI, fig. 2.
This species, which is a new one and also represents a new genus,
is from the upper Devonian of Wildungen, Germany. It was placed
at my disposal by Professor O. Jaekel of Greifswald. On the labels
accompanying it Professor Jaekel had named it Epipetalichthys wild-
ungensis. So far, only a single specimen, represented by the imper-
fectly preserved neurocranium, has been found of it. To Judge from
this specimen, the species must have attained about the same size as
Macro pet alichthys rapheidolabis.
PRIMORDIAL NEUROCRANIUM
General remarks. The primordial neurocranium, if we first turn
to this, is partly rather badly crushed and fractured, and, in addition,
also, weathered in places, so that it is not by far as well adapted for a
Macropetalichthyids and other Arthrodires — Stensio 153
detailed examination as that in the Field Museum specimen of Macro-
petalichthys rapheidolabis. In its general shape it must obviously have
resembled that of the last-mentioned species, but, as we shall see, it
differs distinctly from this in certain details.
Its degree of ossification was considerably less than that in M.
rapheidolabis and less also than in M. agassisif, a fact which deserves
a special attention, as we are here concerned with an upper Devonian
form, while the other two just mentioned specimens are both from
earlier divisions of the Devonian. At least this is beyond question the
case with M. rapheidolabis, which occurs only in middle Devonian strata.
On account of the imperfect state of preservation in which the
primordial neurocranium is found, it is not possible to make out with
full exactness the extension of its different layers of substitution bone,
but the following may give an idea of the most important differences
from M. rapheidolabis in this respect.
1. There is no evidence of the labyrinth bone layer. 2. The inner
bone layer seems to be entirely lacking, except in the posterior, narrow
division of the occipital region (text fig. 17). 3. The external bone
layer surrounds the posterior, narrow division of the occipital region,
and extends from this forward also on to the lateral and ventral sides of
the anterior division of the same region and the ventral side of the
labyrinth region. Whether, however, it continued from the latter region
to the ventral side of the orbitotemporal region, cannot be decided, as
the ventral parts of this region have been destroyed. On the ethmoidal
region it is, however, found again on the ventral side and, in addition,
at least partly, on the lateral sides and probably also on the anterior
end. Accordingly, it is lacking on the larger anterior part of the dorsal
side, quite as in M. rapheidolabis, but, contrary to what is the case in
that form, also probably on the lateral sides of the entire labyrinth and
orbitotemporal regions and the posterior side of the ethmoidal region.
If it was lacking on the ventral side of the orbitotemporal region, this
region would obviously have consisted entirely of cartilage. 4. The
canal layers seem to have become entirely reduced.
As far as I can see, there are no traces of ossification centers either
in the occipital region or in the ethmoidal region.
Microscopic Structure of the Bone
The substitution bone of M. rapheidolabis has generally been much
infiltrated by a dark bitumen that is difficult to remove, and, accord-
ingly, no good microscopic sections can be made from it. I could,
however, see rather distinctly that it has numerous cell spaces (PI.
XXXI, fig. 1). In the species under consideration the bone tissue is
i54 Field Museum of Natural History — Geology, Vol. IV.
also not favorably preserved for microscopic investigation, but it is
fully obvious that it there too contained cell-spaces in fairly large
number. In the posterior division of the occipital region we can
further see that from the external and internal bone layers, numer-
ous, irregular trabecular (PI. XXXI, fig. 2) extend a short distance
into the parts of the cranial walls that were occupied by cartilage, so
that there really is certain evidence of cancellous bone, although this
bone was very slightly developed. By means of short bone trabecular
of about the same kind as those just mentioned, the bone layer sur-
rounding the notochordal space (ch.l, PI. XXXI, fig. 2) is connected
with the internal and external bone layers. Otherwise, no details can
be observed.
Occipital Region
As in M. rapheidolabis and M. agassizi? the occipital region is
composed of two divisions, a posterior, long and narrow one, and an
anterior, short and broad one.
ft
A cent
Fig. 17. Epipetalichthys wildungensis Jaekel
Transverse section through the anterior half of the posterior division of the
occipital region. Layers of substitution bone with strong black lines, cartilage
dotted. The trabecular of substitution bone penetrating inwards into the cartilage
from the bone layers not drawn. XVs
ham, haemal groove; ch, space for the notochord; cv, cavum cerebrale cranii.
The posterior one (text fig. 18; PI. XXVIII, fig. 2) is represented
merely by an anterior part, and no certain statement can therefore be
given of its length. It is narrow and, contrary to that in M. rapheido-
labis and M. agassizi f, it equals the other divisions of the primordial
neurocranium in height. Further, it differs from that in the two species
mentioned in the fact that it does not have the slightest traces of the
crista occipitalis lateralis, which in them is such an important feature.
The haemal groove (hcem, text figs. 17, 18; PI. XXVIII, fig. 2) is
rather distinct and it is noteworthy that we find in the part of it pre-
Macropetalichthyids and other Arthrodires — Stensio 155
served two rather shallow depressions (seen in text fig. 18), which,
perhaps, may be segmentally arranged. Otherwise I only wish to
emphasize that the division was narrowest anteriorly at the transition
to the anterior division, gradually increasing somewhat in breadth back-
ward from this place.
A transverse section through the part of the division present (text
fig. 17) shows that the cavum cerebrale cranii (cv) within this part
was very high and narrow and that there was below the cavum cere-
brale a space for the notochord (ch). This space is, like that in Macro-
pctalichthys raphcidolabis, bounded by a bone layer of its own, a bone
layer that is continuous dorsally with the inner bone layer (bottom of
the cranial cavity), and ventrally with the external bone layer (bottom
of the haemal groove), as already pointed out above (PI. XXXI, fig. 2).
No traces of canals for spino-occipital nerves could be seen in the
part of the division present.
--i--c.rv.1
Fig. 18. Epipetalichthys wildungensis
A posterior part of the primordial neurocranium from the central side. X$4-
c.rai, c.rat, anterior and posterior openings respectively of the canal for the radix
aorta: (lateral dorsal aorta); ham, haemal groove; s.ra, groove for the radix
aortae lateral dorsal aorta, anterior of the canal era.
Of the anterior broad division of the region only the ventral parts
are accessible for investigation (text fig. 18; PI. XXVIII, fig. 2). As
far as can be seen from these its shape must have been very much the
same as in Macropetalichthys rapheidolabis.
With regard to the details, we find that in this division the pos-
terior part of the groove for the radix aortae forms a closed canal in
the primordial neurocranium. This canal has its anterior and posterior
openings (c.ralt c.ra2) situated as shown by text fig. 18 and PI. XXVIII,
fig. 2.
Nothing is so far known concerning the vagus canal and its branches.
Labyrinth and Orbitotemporal Region
The labyrinth region (PI. XXVIII, fig. 2) is somewhat longer than
in M. rapheidolabis. It is very fragmentarily preserved, only parts of
the external bone layer of its ventral side being left.
156 Field Museum of Natural History — Geology, Vol. IV.
The orbitotemporal region is still more imperfectly preserved, and
the only statement that can be made about it is that it certainly was
rather short and that the orbits are situated far apart and in such a
way that the eyes must have been directed much upwards, quite as in
M. rapheidolabis.
Ethmoidal Region
On account of the fact that, as pointed out, it has the external bone
layer persisting to a considerable extension, the ethmoidal region has
been fairly well preserved. A large part of its ventral side, an anterior
part of its lateral side, an anterior part of its dorsal side and the
anterior end are available for investigation (text fig. 19; PI. XXVIII,
fig. 1; PI. XXIX, fig. 3; PI. XXX, fig. 1).
From its broad, posterior end, it becomes rapidly narrow forward,
but does not taper to a point, its anterior end being truncated and
rather concave. The breadth at this end is about one-third of that at
the posterior end.
From the antero-ventral part of each lateral side the olfactory cap-
sule (olf, text fig. 19; PI. XXIX, fig. 3; PI. XXX, fig. 1) projects
laterally exactly as in M. rapheidolabis. Unfortunately it is not so well
preserved as in that species, its anterior and external parts being prac-
tically destroyed, and on account of this nothing can be said with cer-
tainty about its external opening and whether there was any nasal fon-
tanels on the ventral side of its lateral part.
The ventral surface of the region (PI. XXIX, fig. 3) is rather
concave in transversal direction and has in the postero-lateral part
somewhat medially of the lateral margin a longitudinal, shallow groove
(s.pal) for the r. palatinus facialis, quite as in M. rapheidolabis. Each
one of the lateral surfaces (PI. XXVIII, fig. 1), is concave in dorso-
ventral direction, and has a long post-nasal portion as in M. rapheidola-
bis. The dorsal surface (text fig. 19; PI. XXX, fig. 1) is exposed
only most anteriorly, the most anterior of the bones in the dermal
cranial roof being weathered away. We find in this part of it a very
deep, anteriorly and dorsally open depression (pfg), which, as far as
can be judged from the conditions in the fossil, seems to have been
lined by the external bone layer.
This depression, which, as already pointed out in the description
of M. rapheidolabis, is obviously the homologue of the similarly
situated depression in the skulls of Elasmobranchs, and which therefore
may be properly called the cavum precerebrale, was, contrary to that
in these fishes, separated from the cavum cerebrale by a thick septum
of cartilage, presumably lined with the external bone-layer on the an-
Macropetalichthyids and other Arthrodires — Stensio 157
terior side. There is thus no fenestra precerebralis. Laterally the
cavum cerebrale is also well separated from the nasal capsules. Its
floor is not perforated by any foramina.
Cavum Cerebrale
Since the internal bone layer, as pointed out, is lacking-, except in
the posterior division of the occipital region, the shape of the cavum
celebrale is known merely in this, where it, as seen from text fig. 17,
is very high and narrow.
The canal for the pineal and parietal organs seems not to have per-
forated the dermal bones of the cranial roof. At least no traces of
such a perforation could be seen in the parts of the dermal bones
preserved.
DERMAL BONES OF THE CRANIAL ROOF
Parasphenoid or other dermal bones were not present on the ven-
tral side of the primordial neurocranium.
The dermal bones of the dorsal side .of the primordial neurocranium
(text fig. 19; PI. XXX, fig. 1) are the same as in M. rapheidolabis,
and their extensions and relations, too, are about the same as in this,
with the exception, however, that the plates Mt and M2 do not meet,
but are separated by the plates Llf which extend to and suture with
one another in the median line.
The plate Ls is, in the posterior part, perforated by a rather large,
oblong foramen (d.end), which certainly must be the external opening
of the canal for the ductus endolymphaticus. This foramen lies defin-
itely behind the place where in M. rapheidolabis the fossa endolym-
phatica is situated, and it seems therefore, rather probable that the duc-
tus endolymphaticus from this fossa extended somewhat backward be-
neath the dermal cranial roof, thus making a bend and continuing back-
ward quite as it does in Chlamydoselachus (Goodey 1910, PI. 43, figs.
7, 8) and several other Elasmobranchs (Retzius 1881). In this con-
nection it deserves also to be emphasized that the position of the ex-
ternal opening of the ductus endolymphaticus in the form under con-
sideration has, in relation to the sensory canal system, exactly the same
position as in Chlamydoselachus (Cf. Allis 1923, PI. II).1
The ornament on the dermal bones of the cranial roof consists of
short, rather low, but nevertheless distinct, ridges, which, on the plates
5" and M2, seem to lie with their longitudinal axis about parallel with
1In Chlamydoselachus the ductus endolymphaticus has its external opening
posterior of the supratemporal commissure of the main lateral lines; in Acanthias,
Mustelus and others, on the contrary, it has this opening anterior of the same
commissure (Cf. Allis 1901, PI. 10).
158 Field Museum of Natural History — Geology, Vol. IV.
the longitudinal axis of the head. On the other bones, excepting the
plate Mlt on which their arrangement is unknown, the ridges are, on
the contrary, arranged concentrically with the edges of the bones (PL
XXIX, fig. 4). In the Macropetalichthys species known so far, the
ornament consists of fairly sparse tubercles, which are, in the main,
irregularly scattered.
THE SENSORY CANALS OF THE HEAD
The sensory canals of the head are known merely in the bones of
the dermal cranial roof, in which they form closed canals. They are
finer than in M. rapheidolabis ; and the ridges for them on the lower
Pff
Fig. 19. Epipetalichthys wildungensis
Head in dorsal view with the anterior parts of the bone plate Mi removed to
show the cavum precerebrale. The sensory canals marked with dotted outlines
and colours. XJ4-
Li, L2, L3, Mi, Mi, Pi, P2, S, dermal bones of the cranial roof (the anterior
larger part of the plate Mi, removed) ; d.cnd, external opening of the canal for
the ductus endolymphatic^ ; ifc, infraorbital sensory canal (blue); Ic, cephalic
division of the main lateral line (red); na, nasal aperture; olf, nasal capsule;
orb, orbital entrance ; pfg, cavum precerebrale ; st.com, supratemporal commissure
between the cephalic division of the main lateral lines (red) ; soc, supraorbital
sensory canal (green).
Macropetalichthyids and other Arthrodires — Stensio 159
sides of the bones are also less strong than in that species. They
open outwards with numerous short and generally unbranched tubuli
(PI. XXX, fig. 1), which, however, contrary to the case in M. rapheido-
labis are arranged like those in M. agassisi? and M. pelmensis in a
single series.
The supraorbital sensory canal (soc, Fig. 19; PI. XXX, fig. 1)
does not extend as far backwards as in M. rap held olabis, and, on this
account, does not meet its fellow of the opposite side in the median
line nor does it anastomose with the commissure (s.com) between the
cephalic divisions of the main lateral lines, but ends some distance
anterior of this. Behind the part where it ends as a closed canal it
may, however, probably have continued a short distance further back-
wards as a pit line. Its anterior parts are not known, but it seems
very probable that anterior of the most anterior part preserved in the
fossil it curved strongly backwards and went first dorsally and then
posteriorly of the nasal opening down to the infraorbital sensory canal,
as it does in Coccosteus and Elasmobranchs. All the tubuli of the
preserved portion of the canal issue in lateral direction, as is clearly
shown by PI. XXX, fig. 1.
The infraorbital sensory canal (ifc, Fig. 19; PI. XXX, fig. 1)
is, in the main, as in M. rapheidolabis.
The cephalic division of the lateral line (Ic, Fig. 19) ; PI. XXX,
fig. 1) has also the same course as in M. rapheidolabis, and is, as in
this, in communication with its fellow of the opposite side by a trans-
versal commissure (st.com, Fig. 19; PI. XXX, fig. 1), which
traverses the plate L3 and the plate M2 to the plate L3 of the opposite
side. This commissure is, however, contrary to that in M. rapheidolabis,
almost straight, and, as already mentioned, it does not communicate with
the posterior ends of the supraorbital canals. It seems even to be
situated behind the point in which the prolongations of these canals
appear to meet each other. Consequently it has such a position and
shape that in M. rapheidolabis it would seem to correspond to the
presumed line of pit organs behind the cross commissural canal and
not to this canal, and it thus appears to really represent the true supra-
temporal commissure of fishes. If this opinion is true, the homologue
of the commissural canal of M. rapheidolabis would either be entirely
lacking or be represented by a paired line of pit organs corresponding
to the posterior head line of pit organs in recent fishes. (Cf. Allis
1889, pp. 505-506; 1903, p. 187; 1905; 1923, pp. 195-201; Herrick
1901, pp. 222-223; Stensio 1921, pp. 218, 263; 1922, p. 192; 1923,
p. 1258; 1925; etc.)
160 Field Museum of Natural History — Geology, Vol. IV.
The most anterior portion of the cephalic division of the lateral line
situated between the supratemporal commissure and the infraorbital
canal, does not open outward and has no tubuli at all. The remaining
portion, on the contrary, is provided with numerous short tubuli, which
issue on the lateral side and in ^lateral direction (PI. XXX, fig. i).
The supratemporal commissure' also has numerous tubuli, but these
seem, as a rule, to perforate the bones in a straight upward direction
(PI. XXX, fig. i).
Whether there was a preopercular canal or not, it is at present
impossible to decide.
Remarks
As is obvious from this description, the form described here as
Epipetalichthys wildungensis is well separated from other Macropeta-
lichthyids known hitherto, and the erection for it of a new genus and
species is therefore fully justified.
SOME GENERAL REMARKS ON THE
MACROPETALICHTHYIDS
With the facts previously known, it has not been possible to arrive
at any certain conclusion concerning the relationships of the Macropet-
alichthyids, but from what has now been determined some deductions
may be made. Thus, there cannot be the slightest doubt that the
Macropetalichthyids are true fishes, and, moreover, it is also clear that
among recent forms they most resemble the Elasmobranchs. The
most important characters which they have in common with these
are the following: (i) The general shape of the primordial neuro-
cranium, especially the tendency to broadening of the ventral surface,
partly at the expense of the lateral surfaces. (This is especially the
case in the labyrinth region in which the sacculus and perhaps the
canalis semicircularis externus had about the same relation to the
ventral surface as in Chlamydoselachus and most other Selachians).
(2) The position and relations of the olfactory capsule. (3) The
presence of a nasal fontanelle on the lower side of the nasal capsule
as in Chlamydoselachus and certain other Selachians. (4) The pres-
ence of the cavum precerebrale. (5) The general shape of the laby-
rinth, especially with regard to the position of certain of its main parts,
as, for instance, the utriculus. (6) The presence of the ductus endo-
lymphaticus and the fact that there probably was a distinct fossa
endolymphatica on the dorsal surface of the primordial neurocranium
beneath the dermal bones. (7) The fact that the ductus endolympha-
Macropetalichthyids and other Arthrodires — Stensio 161
ticus perforated the dermal cranial roof and had an external opening
situated as in certain primitive Selachians (Chlamydoselachus). (8)
The general shape of the brain as far as this can be restored from the
exit of the nerve canals and the shape of the cavum cerebrale. (9) To
a certain extent the course and arrangement of the blood-vessels and the
presence of certain important trunks, as, for instance, the vena hyoidea.
(10) The fact that the palatoquadrates as far as can be judged did not
articulate with the ethmoidal region but must have been suspended be-
low this by ligaments. (11) The probable course of the anterior part
of the supraorbital sensory canal. (It should also be mentioned that
the sensory canals must have had rather numerous sense organs and
that they opened outward with very many tubuli.)
There are, of course, many differences between the Macropetalich-
thyids and Elasmobranchs, but these differences seem in general from
the morphological point of view to be of much less importance than
the agreements. The fact that bone occurs to a considerable extent in
the Macropetalichthyids, while it is, as we know, totally lacking in
recent Elasmobranchs, ought at first perhaps to be considered as an
important difference, but from what is now known of the occurrence
of bone in the vertebrate series, even this must be considered to be
a character of minor importance. In order to illustrate this the fol-
lowing facts are given :
Typical bone tissue is found in the Cephalaspids, which, according
to my recent investigations not yet published, are lowly organized
vertebrates, without jaws, with three distinct prootic branchiomeres
(hyoid segment, mandibular segment, and premandibular segment) and
indications of still more. Further, there are in them also certain evi-
dences that there were two segmentally arranged nerves anterior to the
facialis, and we are able to conclude that they were in these respects
much more primitive even than the recent Cyclostomes. As, in addition,
more or less osteoid tissues occurs in a number of other very lowly
organized Ostracoderms, we conclude that bone must be a tissue
that appeared very early and very low down among the vertebrates,
in fact already in the common stem of fishes and Cyclostomes.
It is also well worthy of notice in this connection that in the Cephala-
spids both the brain and the ears were situated in the bone of the
cephalic shield, which means that there is in them no difference be-
tween dermal bone and substitution bone.
In the earliest appearing Dipnoans, Crossopterygians and Actin-
opterygians the primordial neurocranium and primordial skeleton in
general are regularly well ossified, but the degree of ossification grad-
1 62 Field Museum of Natural History — Geology, Vol. IV.
ually decreases during the course of the geological periods, the sub-
stitution bone in many forms being finally entirely lost. We thus
find that the primordial neurocranium in the Devonian Dipnoi
Scaumenacia (my own observations) and Dipterus (Traquair 1878,
p. 5; Watson and Day 1916, p. 33; observations made by the author
in the Royal Scottish Museum, 'Edinburgh) is very well ossified, while
it is not found to be ossified in any of the Carboniferous or post-
Carboniferous forms, if we except the small ossification in the
most posterior part of the occipital region of the recent Ceratodus (Cf.
Watson and Gill 1923 ; K. Furbringer 1904, PI. 38, fig. 7 ; Teller
1891 ; etc.). Among the Crossopterygians the process of reduction
of the bone tissue is especially well displayed in the Coelacanthida, in
which the Devonian form Diplocercidcs has the primordial neurocran-
ium comparatively well ossified (Stensio 1922; 1923, pp. 1259-1269),
while the post-Devonian forms all have it to a very considerable extent
consisting of cartilage, the bone being limited only to certain spots
(Stensio 1921, pp. 53-61, 91-94, 120-122; Watson 1921 ; cf. also
Stensio 1923, pp. 1241-1259). According to information which I
have received from Professor D. M. S. Watson of London, the pri-
mordial neurocranium is, further, much more ossified in the Carbonifer-
ous Palseoniscids than in the Triassic ones Birgeria and Acrorhabdus
(Stensio 1921, pp. 152-175, 182-186, 187-198, 211, 224, etc.; pp. 238-
250). Similar conditions are also met with in the Saurichthyids, in
which those from the lower Triassic have their primordial skeleton
very completely ossified, while primordial bone seems to be almost en-
tirely lacking in those from the lower Jurassic (Stensio 1925). In
the Chondrosteids, Acipenserids and Polyodontids, which are very
closely related to the Palaeoniscids and Saurichthyids, the primordial
neurocranium, as we know, is, as a rule, without bone. (We find
bones only in very old specimens which indicates that the bone has
lost its importance and that the parts still remaining therefore develop
onto-genetically very late.) Finally, in a newly discovered ganoid
from China, which with regard to its cranial anatomy much resembles
Amia, the primordial skeleton is much more ossified than in Anna.
A similar reduction affects in several forms also the dermal bones.
In the Dipnoi from the Devonian the dermal bones all lie superficially,
are ganoine-covered and well developed (Cf. Watson and Gill 1923).
In many post-Devonian Palaeozoic Dipnoans on the contrary, we see
that, for instance, the most anterior and posterior bones of the dermal
cranial roof sink down beneath the surface of the dermis, lose their
ganoine covering and the grooves for head lines of pit organs, and, in
Macropetalichthyids and other Arthrodires — StensiS 163
the recent Ceratodus, this procedure has gone so far that the anterior
and posterior parts of the dorsal side of the primordial neurocranium
are uncovered by dermal bones (Cf. Teller 1891, Figs. 2-4; cf. also
K. Furbringer 1904, pp. 498-500, who strangely enough arrives at a
quite opposite conclusion). The same procedure as the one described
in the Dipnoi has taken place in several other Teleostomes, as, for in-
stance, in the Chondrosteids, the Acipenserids and Polyodontids, which
all are certainly descendants from forms with a very completely de-
veloped dermal skeleton.
From the facts now given we thus find that most groups of Tele-
ostomous fishes in which the skeleton has been investigated in detail,
undoubtedly appear to be degenerating with regard to the degree of
ossification. And in this connection it is worthy of notice that the
Macropetalichthyids themselves too seem to represent a degenerative
series in this respect, as the primordial neurocranium of the middle
Devonian Macropetalichthys rapheidolabis is much more completely
ossified than that of the upper Devonian Epipetalichthys wildungensis.
If we turn to the Elasmobranchs, it has generally been considered
that these do not have real bone tissue. This may be true for the
more typical later forms, but if we go back to the Acanthodians the
conditions seem to be different. Reis (1896, pp. 179-184, PI. VII,
fig. 1) and with him most other authors certainly say that in these too
there is no bone, but that the primordial skeleton is calcified. But, as
is evident from Reis' description and figures, the manner of "calcifica-
tion" is there of quite another kind from that in the typical Elasmo-
branchs and in reality everything indicates that in the so-called calcified
cartilage of Acanthodians we are concerned with true bone. That
Reis1 did not recognise it as bone is probably due simply to the fact
that he expected to find the bones of fishes with the structure occurring
in higher vertebrates, i.e., throughout, or almost throughout, with a
lamellary structure and the lamellae in the interior of the skeletal ele-
ments surrounding Haversian canals. This, however, is often not
the case, the bone of lower vertebrates generally being entirely without
Haversian canals and lamellae and even rather often without cells.
Unfortunately the knowledge of the microscopic structure of bone in
'According to Reis the primordial skeleton of the Acanthodes consists peripher-
ally of a rather lamellary layer containing numerous cell spaces. Internally to
this follows a cancellous layer and most centrally a vacuity occupied in the fossil
by stone but which in the living animal certainly consisted of cartilage. In the
cancellous layer there are, if Reis is correct, no cell spaces. A microscopic inves-
tigation of the primordial skeleton of the Acanthodians, from the new points of
view advanced here would be very desirable, but unfortunately I had no mate-
rial with which to undertake one myself.
1 64 Field Museum of Natural History — Geology, Vol. IV.
lower vertebrates is still very imperfect. The conditions in several
other Elasmobranchs from the Palaeozoic also seem to indicate that
the Elasmobranchs in general originally had an osseous skeleton (Cf.
Woodward 1924). Finally, it ought, in connection with this also, to
be pointed out that, according to the recent interesting investigations by
Weiden reich (1923, pp. 4 1 5-4I 9, 461-462), there is really no funda-
mental difference between calcification and ossification.
From what we now know of the occurrence of bone in the vertebrate
series, we must evidently presume that bone must have occurred also
in the ancestors of and in the most primitive Elasmobranchs and the
fact that it is to be found in the Acanthodes is therefore only a matter
of course.
That the Macropetalichthyids cannot be closely related to any
Teleostomous fishes so far known is thus, despite the presence of bone
in their skeleton, beyond question. The development of the dermal
bones in their cranial roof and the lacking of dermal bones on the
ventral side of the primordial neurocranium, together with the Elasmo-
branchian characters, show definitely that they are not specialized
Teleostomes at all but forms that among fishes known hitherto are in
some way most closely related to the Elasmobranchs. In fact the indi-
cations as far as we can judge at present seem to be that the Macro-
petalichthyids evolved from some early, very primitive Elasmobranchian
form soon after the Elasmobranchian stem had separated from the
stem of Teleostomous fishes.
CERTAIN REMARKS CONCERNING THE OTHER NON-
MACROPETALICHTHYID ARTHRODIRES
The increased knowledge that we now possess of the anatomy of
the Macropetalichthyids throws of course new light on the other
Arthrodires too, and it is therefore of importance to give here some
remarks on these. I shall begin with the Phlyctenaspids, which, as
Jaekel has pointed out (1911, p. 46) seem to be most nearly related
to the Macropetalichthyids. I shall then turn to the Coccosteids,
Homosteids, Mylostomids, Ptyctodontids and, finally, to a form which
has not hitherto been recognized as an Arthrodire.1
1A satisfactory classification of the Arthrodires into families is difficult to
give at present on account of the imperfect knowledge we still possess of the
various forms, and because the opinions of different authors are widely at vari-
ance in this respect. The subdivisions given by me here are merely intended to
simplify the account of certain anatomical features and not to give positive
conclusions as to the mutual relations of all the forms.
Macropetalichthyids and other Arthrodires — Stensio 165
THE PHLYCTENASPIDS
To the Phlyctenaspids are here referred among others the genera
Phlyctenaspis and Acanthaspis (Arctolepis, Eastman 1908 b; Kaer
1916).
Primordial Neurocranium
Of the primordial neurocranium no remains have been found so
far, a fact which indicates that it was either entirely cartilaginous
or at least very slightly ossified. Despite this, it is, however, possible
with the help of the configuration of the dermal cranial roof and the
conditions in the Macropetalichthyids, to obtain an idea of its general
shape, at least in the anterior parts. (See Fig. 20.) It is thus obvious
that the ethmoidal and labyrinth regions must both have been very
short and broad and that the labyrinth region was rather long, in any
case several times as long as the two first-mentioned regions. It
seems probable also that the occipital region was rather long, too. The
differences with regard to general extent of the various regions from
those in Macropetalichthys rapheidolabis can be clearly seen by com-
paring Fig. 20 with Figs. 1, 3, 5, etc.
Fig. 20. Phlyctenaspis acadica
Restoration of the outlines of the primordial neurocranium with the guidance
of the shape of the dermal cranial roof (Cf. Fig. 21) and the conditions in
Macropetalichthys rapheidolabis. Shape of the cavum cerebrale indicated by
broken lines. X^-
Olf, olfactory capsule; orb, orbit; par, pineal opening.
1 66 Field Museum of Natural History — Geology, Vol. IV.
By the use of broken lines I have tried in Fig. 20 to indicate
the possible shape and extension of the cavum cerebrale cranii, which
if my view is correct ought to have been fairly like that in Macropet-
alichthys except that the canals for the tractus olfactorii must have
diverged very strongly and rapidly from each other. As the anterior
median dermal bone plate M (Fig. 21) has on the lower side close
to its posterior border a pronounced pit corresponding to the pineal
pit in Titanichthys, we are quite sure of the position of the dorsal
part of the canal par, and from this we are of course able to conclude
approximately where the anterior end of the cavum cerebrale cranii
was situated. Whether there was any distinct cavum precerebrale or
not it is not possible to decide, and, finally, it may be added that no
external opening of the canal for the ductus endolymphaticus has hith-
erto been observed.
Fig. 21. Phlyctenaspis acadica
Dermal cranial roof. Sensory canal grooves with dotted outlines. Ornament
not drawn. Sketch made after specimen P. 6555 in the British Museum, Lon-
don. XVa-
A, B, I, L, M, Pi, Pi, S, dermal bones of the cranial roof ifc, infraor-
bital sensory groove (blue) ; Ic, cephalic division of the lateral line (red) n,
nasal aperture; orb, orbital entrance; poc, preopercular sensory groove (yellow) ;
soc, supraorbital sensory groove (green), t, line on the plate M, perhaps a vestigial
suture.
Macropetalichthyids and other Arthrodires — Stensio 167
Dermal Cranial Roof
The plates Plt P2, and S (Fig. 21) correspond, at least in the
main, to the plates in Macropetalichthyids lettered Plt P2, and 5"
(Figs. 15, 19), while the plate / certainly represents a posterior part
of the plate M2 of the latter. The plate B (Fig. 21) seems to
include the homologues of the plates L2 and L3, in the Macropetalich-
thyids, together with a part of the plate M2 in these. The plate A
(Fig. 21) is, as far as can be seen at present, represented in the
Macropetalichthyids by the plate Lx, and an anterior part of the plate
M2. The plate L and the most anterior part of the plate M, seem to
be without homologues in Macropetalichthyids and that in this case
we are concerned with reductions in the last-mentioned forms is highly
probable from the conditions in the other Arthrodires. The plate M
has probably arisen by the coalescence of two plates, an anterior and a
posterior one, homologous to the rostral and pineal plates respectively
of the Coccosteids (Fig. 24) as it is crossed by a fine transverse line
which seems to be the vestigial suture between the presumed two com-
ponents.
As far as can be judged at present from the different conditions of
their dermal cranial roofs, the Macropetalichthyids and Phlyctenaspids
must both have descended from primitive Arthrodires in which the
medial parts of the cranial roof were occupied by a larger number of
bone plates than in any Arthrodire known hitherto.
Between the plates L and Px there is in the fish a rounded notch
(orb, Fig. 21) indicating the position of the certainly very small
orbital entrance, which is thus in relation to the dermal cranial roof
situated in another manner than in the Macropetalichthyids and which
in addition appears to be directed much more laterally than in these.
Concerning the ventral boundaries of the orbital entrance nothing
certain is known at present. Between the antero-medial border of
the plate L and the postero-lateral border of the plate M, there is
found another notch in the outline of the dermal cranial roof, a notch,
which as far as I can see, must be the external nasal aperture (n, Fig.
30.
Most of the dermal bones are ornamented with tubercles and must
have had a superficial position. Only the most anterior bones L and
M seem to have been situated somewhat deeper in the skin, as at
least their anterior parts usually seem to be without ornament and the
supraorbital sensory canal anterior of the plate A probably passed
entirely externally of the plate L.
1 68 Field Museum of Natural History — Geology, Vol. IV.
Sensory Canal System of the Head
The sensory canals are known only on the dermal cranial roof,
on which they form open grooves. These grooves are, however, at
least in several cases, deep and narrower at their external opening
than further inwards (Fig. 22V so that they in fact open outwards
only with a narrow slit. Concerning the number of sense organs
in the grooves nothing can be said at present.
The supraorbital groove (soc, Fig. 21), which is very short,
has its posterior end at about the ossification centre of the plate A.
From this point it passes forward and laterally to the antero-lateral
corner of the plate. On the plate L, there are no traces of it, but
it certainly continued still further forwards, although situated there
entirely externally of the plate L in the most external layers of the
skin. From the direction of the part known of the groove it seems,
as is obvious from text fig. 21, certain that the other hitherto unknown
part of the same groove passed, as has already been emphasized above,
antero-latero-ventrally posterior of the nasal aperture as it does in the
Coccosteids and Elasmobranchs.
Fig. 22. Sketch of a sensory canal groove of a Phlyctenaspid in transversal
section. Magnified.
The infraorbital groove (ifc, Fig. 21) is, with its posterior
end, which is situated approximately at the ossification centre of the
plate P2, continuous with the cephalic portion of the lateral line groove.
It goes forward to about the ossification center of the plate Plt where
it suddenly bends latero-ventrally and leaves the cranial roof posterior
to the orbital entrance.
Just at the bend, it is continuous with a groove (ifbr, Fig. 21)
which goes in an arch postero-medially to near the centre of the plate
B. This groove must, as is easily understood, have been innervated
either from the n. ophthalmicus lateralis and represent a posterior
part of the supraorbital sensory canal, or from the r.oticus lateralis,
in which case it must be considered as a branch from the infraorbital
groove. Which one of these two alternatives is the true one it is
difficult to decide from the conditions in the Phlyctenaspids, but when
we come to the Coccosteids we find there, as we shall see, certain facts
which seem to be much in favour of the latter. I have therefore
called the groove in question the postero-medial branch of the in-
Macropetalichthyids and other Arthrodires — Stensio 169
fraorbital groove. A further support for this view is given perhaps
by the conditions in Lcemargus. In this form the infraorbital canal
with its posterior part on the dorsal side of the head, turns abruptly
medially and continues in this direction till it almost meets its fellow
of the opposite side in the median line (Ewart 1892, PI. I, fig. 1).
A groove issuing in postero-medial direction from the in-
fraorbital groove quite in the same manner as the postero-medial
branch of the infraorbital groove in the Phlyctenaspids occurs in many
Stegocephalians (Wiman 1914, PI. I, figs. 1, 2, 3; PI. Ill, fig. 2; PI.
IV, figs. 2, 6, 7; PL. V, fig. I; PI. VII, in the form figured on this
plate the branch in question is double; 1916, PI. XV, cf. Moodie 1908;
1915). A homologue to it is also found in larval forms of Triton,
in which, however, like the whole sensory canal system, it is repre-
sented merely by isolated pits arranged in a line.
The cephalic division of the lateral line (Ic, Fig. 21) is, as
already pointed out, like the other sensory canals, an open groove.
Its anterior end is situated as the ossification centre of the plate P2,
in which it is continuous with the infraorbital groove. It goes back-
ward to the postero-lateral edge of the plate S, its farther course back-
ward being unknown. There is no cross commissural groove, which
connects it with its fellow of the opposite side and, as far as known,
there is not even any pit line representing a commissure of this kind.
The preopercular groove (poc, Fig. 21) is represented by a
dorsal part which issues in a postero-latero-ventral direction from the
point at which the infraorbital groove and the lateral line groove (Ic)
meet one another.
Nothing is known of pit lines, but this may perhaps be due to the
unfavorable state of preservation in which remains of Phlyctenaspids
are generally found.
Dermal Covering of the Trunk
The anterior parts of the trunk of the Phlyctenaspids are, as we
know, enclosed in a strong armour, which, as it follows immediately
behind the head, obviously, at least with its anterior parts, occupies
the position of the dermal shoulder girdle of the TeleostomoUs fishes.
A further support for this view is the fact that, as set forth in the
description of M. raphcidolabis, the branchial basket cannot have ex-
tended backward into the abdominal armour, but must have been
limited entirely to the head. The powerful Elasmobranchian-like spine
that in all Phlyctenaspids is rigidly attached to the antero-lateral edge
of the antero-ventro-lateral and the lateral edge of the inter-lateral
170 Field Museum of Natural History — Geology, Vol. IV.
of each side, thus has in fact the approximate position of the pectoral
fin of ordinary fishes and seems to me to represent either the entire
dermal skeleton of this fin or to be a spine that was attached to its
anterior border while the other parts of the fin have become reduced.
THE^TOCCOSTEIDS
To the Coccosteids in the broad sense in which they are taken
here are referred not only Coccostens, Pachyosteus, Rhinosteus, Pholi-
dosteus, Chelyophorus, and others, but also such genera as Dinichthys,
Titanichthys, and Selenosteus, the chief reason for this being the
characters of their dermal cranial roof.
Primordial Neurocranium
In Chelonichthys primigenius (Eichwald i860, pp. 1526-1527 PI.
57, figs. 1, 2) the primordial neurocranium was partly well ossified,
but unfortunately no details are known of it. In other forms it seems
as a rule to have been entirely cartilaginous or at least almost entirely
so, for no certain remains of it have been found so far.
Turning first to Coccosteus decipiens and the forms closely allied
to it, we can at once understand from the proportions of the dermal
cranial roof that the ethmoidal and occipital regions of the primordial
neurocranium in them must have been short, while the remaining two
regions, the orbitotemporal and labyrinth regions, had a rather con-
siderable length. It deserves also to be mentioned in this connection
that the primordial neurocranium must have been fairly high, in any
case higher than in the dorso-ventrally much flattened Macropetalich-
thys rapheidolabis. That this was the case is easily understood if we
look at the head, for instance, of Pholidosteus friedelii (Jaekel 1919,
Fig. 2) from the lateral side. In Fig. 23 A, the general shape of
the primordial neurocranium has been drawn as I suppose it to have
appeared. In this figure also the outlines of the cavum cerebrale
cranii and the canals for the olfactory tracts have been put in mainly
with the guidance of the conditions in M. rapheidolabis and the posi-
tions of the nostrils and the pineal pit on the lower side of the pineal
plate {Pi, Fig. 24). We see from this that the canals for the ol-
factory tracts cannot have diverged quite so strongly forward as in the
Phlyctenaspids.
While in Coccosteus and its nearest allies the lower side of the
dermal cranial roof is rather smooth, or in any case without prominent
ridges or formations of this kind, it has in Dinichthys a paired lamella,
which extends some distance downwards. The position of this lamella,
Macropetalichthyids and other Arthrodires — Stensio 171
which in places is thickened or is provided with downward di-
rected processes or is irregular in other ways, is shown by Fig. 2,
PI. IV, and Fig. 1, PI. LII of Newberry's description in 1889, and by
Fig. 4 D of Woodward's description in 1922, and, as we understand
from these figures, it must have covered a dorsal portion of the outer
surface of each lateral wall of the primordial neurocranium. Accord-
ingly it shows the outlines of the dorsal side of the primordial neuro-
cranium, a fact that was recently pointed out by Woodward (1922,
P- 33).
With the guidance of this lamella and the conditions in the
Macropetalichthyids, the restoration shown in Fig. 23 B was made.
As we see from it, the primordial neurocranium of Dinichthys agrees
in several points with that in the Phlyctenaspids and Coccosteus but
Fig. 23. Restoration of the outlines of the primordial neurocranium. A, of
Coccosteus; B, of Dinichthys. The possible extension of the cavum cerebrale
cranii and the olfactory tracts is indicated by broken lines, co, occipital condyles ;
olf, olfactory capsule; orb, orbit.
differs, however, distinctly at least from that in the former by the
greater length and comparatively less considerable breadth of its orbi-
totemporal region. Further, it is also worthy of notice that the orbito-
temporal region probably was not definitely bounded from the labyrinth
region, but that the anterior parts of this were rather narrow too, while
its posterior parts were very broad. Whether the short occipital region
had an anterior broad division as in the Macropetalichthyids it is riot
possible to say, as we do not know how the vagus canal had its course.
According to the opinion advanced by Woodward (1922, p. 33)
the primordial neurocranium in Dinichthys would not have extended
farther back beneath the dermal cranial roof than approximately to
172 Field Museum of Natural History — Geology, Vol. IV.
the transversal plane through the posterior ends of the supraorbital
sensory canals (cf. Fig. 24 B). In the Macropetalichthyids the
posterior ends of the supraorbital canals are, as is seen from PI. XX in
the present paper situated at a place about midway between the laby-
rinth cavities. In other fishes, too, ' they do not reach much farther
back, even in those cases whenuiey with certainty include the anterior
head lines of pit organs. This fact in connection with the circum-
stance that the paired lamella on the lower side of the dermal cranial
roof distinctly continues postero-laterally of this place to the very
postero-lateral corner of the dermal cranial roof, seems to me to indi-
cate with almost absolute certainty that the primordial neurocranium
of Dinichthys extended backward as far as the dermal cranial roof
and that the posterior parts of the labyrinth region were as broad as
shown by my restoration in Fig 23 B.
The paired bony lamella on the lower side of the dermal cranial
roof in Dinichthys is held by Woodward to be an ossified dorso-
lateral part of the lateral wall of the primordial neurocranium and we
would thus have here a primordial component coalesced with the der-
mal cranial roof. That this perhaps is the case seems probable to me
especially from the conditions in the Coelacanthids, in which a part of
the primordial skeleton in certain specialized Triassic forms has
coalesced with an overlying dermal bone, while in the other forms it
has retained its original primitive relations (Stensio 1921, pp. 53, 93,
97, 124; 1923, p. 1269; cf. Allis 1899; 1909a). In the Coelacanthids
this coalescence between dermal bone and primordial bone has, how-
ever, obviously taken place in connection with a process of reduction
of the bone of the primordial neurocranium, and, from what has been
emphasized above there is every reason to believe that this is also the
case in Dinichthys. What the paired bone is that Newberry (1889,
pp. 147-148) describes from the interior of the head in Dinichthys
tcrelly, it is not possible to decide. Perhaps it may be an ossified part
of the primordial neurocranium.
A common character for all the Coccosteids is the fact that their
orbit usually was large and that the eyes must have been directed
straight or almost straight laterally.
Dermal Cranial Roof
The dermal cranial roof of Coccosteus decipiens (Fig. 24 A; 25)
is rather similar to that in Phylyctenaspis (Fig. 21), but differs from
this by the somewhat different shape of the various bones and the fact
that the rostral plate (R) and the pineal plate (Pi) are always inde-
Macropetalichthyids and other Arthrodires — Stensio 173
pendent. Further, it is also to be noticed that the orbital entrance is
bounded dorsally by the plate A, while in Phlyctenaspis it is bounded
there by the plates L and Plf the plate A there lying medially of the
two just-mentioned plates. The external nasal aperture is paired, sit-
uated rather close to its fellow of the opposite side and directed almost
straight forward. It is bounded medially by the rostral plate (R),
laterally by the plate L.
The upper Devonian Coccosteids are generally more specialized
than Coccostcus decipiens. (Cf. Jaekel 1906a; 1907; Dean 1901,
figs. 14-18; Eastman 1908a, pp. 89-156; 1908b, pp. 152-205;
Bryant 1918, pp. 26-105; Woodward 1922, fig. 4, pp. 30-36.)
The ornament of the bones in them is generally fine or is even in certain
cases lacking; the bones have become firmly united to one another and
in the American forms, as, for instance, Dinichthys , Titanichthys and
Stcnostens, the plate L has hitherto not been found. This indicates
either that it had sunk deep down in the skin and lost its connections
with the neighbouring plates or that it had become entirely reduced, as
in the Macropetalichthyids (Cf. Woodward 1922, p. 35).
In the restorations of the Dinichthy s-species exhibited in various
American museums, no consideration seems to have been given either
to the absence of the paired plate L or to the circumstance that the
primordial neurocranium with the olfactory capsules must have ex-
tended somewhat forward beyond the anterior margin of the plates A
(Fig. 24 B) of the dermal cranial roof. (Cf. the restoration of Dean
1909, Pis. 38-39; and by Bryant 1918, PI. 1, both of which are
incorrect in this respect.)
Dermal Bones of the Cheek
In Coccosteus decipiens and its nearest allies, the cheek is regularly
occupied by two bones, a large, anterior infraorbital (So, Fig. 24),
which forms the ventral boundary of the orbital entrance and a small
plate (Ps, Figs. 24, 25) \ which is situated behind the infraorbital and
occupies the most posterior part of the cheek. (Cf. Traquair 1890,
PI. 10, figs. 1, 2; Jaekel 1902, Fig. on p. 107; 1919, Fig. 2, Qj.) In
the American forms from the upper Devonian the last mentioned plate
is, as far as I know, always lacking, a fact which probably indicates
that it has become reduced. (Cf. Eastman 1908a, Figs. 23, 24, 29;
1908b, Figs. 27, 31; Hussakof 1906, Figs. iB, 12; Dean 1909, Fig.
56, Pis. 38-40, etc.)
xIn certain cases this plate may probably be subdivided into two (Jaekel 1902,
Fig. on p. 107).
174 Field Museum of Natural History — Geology, Vol. IV.
Visceral Skeleton
The primordial visceral skeleton seems generally to have consisted
chiefly of cartilage, but in Pholidostcus (Jaekel 1907, pp. 176-186;
1919, pp. 77-87, Figs. 4, 8) and Erromenosteus (Jaekel 1919, pp. 84-
86, Fig. 9) it is certainly at least partly ossified.
In Dinichthys intermedins Woodward has found a bone (Wood-
ward 1922, p. 35) which seems to belong to the palatoquadrate, but
whether this bone was an ossification in the palotoquadrate cartilage
itself or whether it is simply a dermal bone developed in relation to this,
it is at present not possible to decide. The occurrence is, however, of
much interest, because it shows that there was in the upper jaw of the
Arthrodires a palatoquadrate medially of the two external elements,
xp
Fig. 24a. Dermal cranial roof
A. Coccostcus decipiens (sketch drawn by the author from specimens in the
Royal Scottish Museum, Edinburgh). For explanation of lettering see next page.
and that the latter two elements on account of this must really be most
nearly comparable with the premaxillary and maxillary respectively.
An additional support for this interpretation of the two external ele-
ments in the upper jaw is' the fact that the anterior one of these ele-
ments in certain species is tuberculated on the external surface (Wood-
ward 1922, p. 33; Eastman 1908a, PI. 7, figs. 4-6) and accordingly
appears to be a superficial dermal bone. .
The Meckelian cartilage has in Pholidosteus and Erromenosteus
(Jaekel 1907, pp. 176-184; 1919, Figs. 4, 8, 9) its pars articularis
ossified, but seems, as far as we know, generally to have been entirely
Macropetalichthyids and other Arthrodires — Stensio 175
cartilaginous.1 On the medial side it was almost entirely covered by
the large, well-known bone (Mix, Fig. 25) which has been termed by
various authors mandibular, gnathal, infero-gnathal, dentary, and
splenial. This bone, on account of its extent and relations, obviously
corresponds to all the dermal bones on the medial side of the man-
dible in the Teleostomes and would therefore if named from this
point of view get a very complex title. For the sake of brevity we
may call it simply the mixicoronoid (Cf. Stensio 1921, p. 244). On
the outside of the postero-ventral part of the Meckelian cartilage, there
is in Pholidostcus (Jaekel 1907, Fig. 5; 1919, Fig. 4) a small dermal
bone which is ornamented with tubercles and which, as pointed out by
Ju*
Fig. 24b. Dermal cranial roof
B. Dinichthys halmodeus (Bones after Eastman 1908a, Fig. 21, and 1908b,
Fig. 24). Sensory grooves according to observations made by the author on a
specimen in Albany, N. Y. Much diminished.
Sensory canal grooves in both A and B, indicated by a double dotted line
and colours.
A, B, J, L, Pi, Pi, Pi, R, S, bones in the dermal cranial roof; Ps, So,
bones of the cheek; ifc, infraorbital sensory groove (blue); ifbr, postero-
medial branch of the infraorbital sensory groove (blue) ; juc, jugal groove
(yellow) ; k, lateral line groove (red) ; tnp, groove, probably for the middle
head line of pit organs (red); na, nasal aperture; orb, orbital entrance; poc,
preopercular groove (yellow) ; pp, groove probably for the posterior head line
of pit organs (red) : soc, supraorbital groove (green) ; xp, branch from the
cephalic division of the lateral line (groove), probably corresponding to a part
of the superatemporal cross commissure (red).
*I have had the opportunity of investigating Pholidosteus friedelii in the col-
lection of Professor O. Jaekel at Greifswald and I fully agree with him in his
interpretations of the elements in the lower jaws of Arthrodires.
176 Field Museum of Natural History — Geology, Vol. IV.
Jaekel, apparently is comparable to the angular of the Teleostomi.
The term angular may therefore properly be employed for it.
As is fully evident from the facts now known, the jaws in the
Arthrodires must have been homologous with those in other fishes, and
their movements, as emphasized by Woodward (1922, p. 35), have
been as normally up and downr (For the previous views on this sub-
ject see: Dean 1901, pp. 105-107; Hussakof 1906; Jaekel 1919,
pp. 96-108; Adams 1919, pp. 123-127.)
In Erromenosteus (Jaekel 1919, Fig. 9) the ventral parts of the
hyoid arch and perhaps also a basibranchial element appear to have
been ossified.
Dentition
Teeth are found in all young specimens of Coccosteids, but while
in certain forms, as for instance, Coccosteus and Diplognatus, they
are retained throughout life, they are in adult specimens of several
other forms such as Dinichthys, Titanichthys etc., entirely abraded by
use, the labial edges of the jaws becoming in these forms transformed
to cutting edges or adapted for crushing.
It is generally maintained (Cf. Woodward 1922, p. 35) that the
teeth occur in a single series on the labial margins of the jaws, but
the investigations undertaken by the author1 show that on the mixi-
coronoid of certain forms there is at least a second and perhaps also a
third or more series of lower teeth medially of the labial ones. All the
teeth are ankylosed to the jaw bones with their bases and generally have
the central parts occupied by a rather large pulp-cavity. In the dorsal
parts this pulp-cavity is generally surrounded by ortho-dentine, in the
basal parts by trabecular-dentine, but in certain teeth the trabecular-
dentine may extend almost to the apex. This seems especially to be
the case in one category of large teeth which belong to the labial
series and are characterized by the fact that they have on their medial
side one, two or three, small, bluntly conical cusps. Basally the tra-
becular-dentine gradually turns into the bone of the mixicoronoid.
Externally of the dentine there is in the distal parts of the teeth a
layer of bright, shining enamel, which is perforated by the distal ex-
tensions of the dentine tubules, these tubules leading to and opening
on the outside of the enamel. Also, the cusps on the medial side of
the large external teeth are provided with an enamel layer of the same
sort. Besides the cusps on the medial side, several of the larger teeth
of the external series have also a small cusp on the anterior and pos-
'The investigations have been made on the mixicoronoid of a Coccosteid from
the Devonian of Spitzbergen.
Macropetalichthyids and other Arthrodires — Stensio 177
terior side, so that, when seen from the lateral or medial side, they
have the appearance of shark teeth (also teeth of Acanthodians).
The occurrence of at least two or three longitudinal series of teeth
in their jaws, together with the circumstance that certain of the teeth
in the external one of these series are shark-like in shape, indicates that
the dentition in the Coccosteids probably has arisen from an Elasmo-
branchian-like one.1 An additional support for this view is the fact
that the medial side of the mixicoronoid ventrally of the longitudinal
series of teeth appears to consist mainly of a dentine-like tissue.
Sensory Canals of the Head
The sensory canal system of the Coccosteids is, as in that of the
Phlyctenaspids, represented merely by grooves on the external sur-
faces of certain of the external dermal bones. The course of these
grooves is best known in Coccosteus dccipiens (Figs. 24 A, 25) and
in the Dinichthys species (Fig. 24 B). The subsequent account is based
chiefly on C. decipietis.
The supraorbital groove (soc, Figs. 24 A, B; 25) goes from the
plate B forward and somewhat laterally to the plate A, which it crosses
to the antero-lateral corner. From this point it continues downward
anterior of the eye and posterior to the nasal aperture on the plate L,
on which it probably anastomized with the infraorbital groove. It is
thus fully evident that with its anterior part it had the same course
as in the recent Elasmobranchs (Cf. Allis 1901, pp. 104-107; 1905).
The infraorbital groove (ifc, Figs. 24 A, B, 25) has on the dermal
cranial roof exactly the same course as in the Phlyctenaspids and has
also a postero-medial branch (ifbr), as in these. We can here see
quite clearly from its transverse course that this branch cannot
represent a. part of the supraorbital groove as one might perhaps be
inclined to think merely from its position in the Phlyctenaspids (Cf .
Pollard 1892b, pp. 546-548). From the dermal cranial roof the in-
fraorbital groove turns latero- ventrally to the cheek, on which it is
first situated on the infraorbital bone. On this it passes first down-
wards and then anteriorly in the normal way, posterior and ventrally of
the orbital entrance. After leaving the infraorbital it continues for-
ward on the plate L, probably anastomizing there with the supraorbital
groove and probably passing anterior of this commissure ventrally of
the nasal aperture. What its relations were at the very anterior end
is not known.
JThe only description that I have found of the microscopic structure of the
teeth in Coccosteids was published by Agassiz 1844 (p. 26). What Agassiz
describes there is probably merely a basal part of a tooth.
178 Field Museum of Natural History — Geology, Vol. IV.
The cephalic division of the lateral line groove (Ic, Figs. 24 A, B,
25) has the same course as in the Phlyctenaspids. Close to its pos-
terior end there issues from its medial side a branch (xp) in anterior
and somewhat medial direction. This branch, which goes close to
the medial edge of the bone-plate 6" and which, contrary to what is
, shown in most restorations, is Tather short and does not reach the plate
B, may perhaps correspond to a lateral part of the supratemporal cross
commissure.
The preopercidar groove (poc, Figs. 24 A, B, 25) issues in the
plate Pi from the point where the infraorbital groove and the lateral
line groove meet each other, quite as it does in the Macropetalichthyids
and Phlyctenaspids. It goes to or close to the postero-lateral corner
of the cranial roof, its passage on the cheek being unknown, as it
probably passed there entirely in soft tissue.
Fig. 25. COCCOSTEUS DECIPIENS
Head and abdominal armour, chiefly from Jaekel (1902). Sensory canal
grooves indicated by double dotted lines and colors.
A, B, J, L, Pi, P2, R, S, bones of the dermal cranial roof ; Ps, So, bones
of the cheek (So, infraorbital bone); Mx, maxillary; Pmx, premaxillary ; Mix,
mixicoronoid ; Spi, spinal; ifc, infraorbital groove (blue); ifbr, postero-medial
branch of the infraorbital (blue) ; juc, /ugal groove (yellow) ; Ic, cephalic
division of the lateral line (red) ; mp, groove probably corresponding to the
middle head-line of pit organs (red) poc, preopercular groove (yellow); pp,
groove probably corresponding to the posterior head-line of pit organs xp, groove
probably corresponding to a lateral part of the supratemporal cross commissure.
About at the place in the infraorbital plate (So) at which the in-
fraorbital groove turns forward from its downward direction, there
issues from it a posteriorly and somewhat ventrally running branch
(juc, Figs. 24 A, B; 25) which obviously is the homologue of the
Macropetalichthyids and other Arthrodires — Stensio 179
horizontal cheek canal of the Elasmobranchs (Allis 1923, pp. 199-
201), the horizontal part of the jugal canal of Crossopterygians
(Stensio 1921, pp. 76-7711923, p. 1258), the horizontal cheek line of
several Teleostomous fishes (Allis 1889, p. 506, PI. 42; 1900, p. 445;
1905, pp. 406-407; 1923 p. 201, Pollard 1892b, p. 548) and the jugal
groove of the Stegocephalians (Moodie 1908, p. 515; 1915, p. 320).
Whether there was any mandibular canal is not known.
Behind the postero-medial branch of the infraorbital groove, too,
paired, short grooves (Figs. 24, 25) are regularly found on the bone
plate B in most of the Coccosteids examined by the author. The
anterior one (mp) of these grooves is situated transversally to the
longitudinal axis of the head and seems to correspond to the middle
head-line of pit organs in the Elasmobranchii and Teleostomi, while
the posterior one (pp) which is situated obliquely so that its anterior
end is nearer to the median line than the posterior one, probably is
homologous with the posterior head-line of pit organs in the same fishes.
(Cf. Allis 1889, pp. 502-509, PI. 42; Herrick 1901, pp. 222-223, PI.
14; Stensio 1921, p. 218; 1923, p. 1258; etc.)
Some Remarks on the Trunk and the Fins1
A pectoral spine (Spi, Fig. 25) although much smaller than in the
Phlyctenaspids, occurs at least in most of the Coccosteids, and it is
beyond question that the paired plate found in Coccosteus decipiens and
Dinichthys gouldi somewhat behind the abdominal armour, really rep-
resents the pelvic girdle (Cf. Woodward 1891, pp. 282, 289; 1922, p.
35; Dean 1896, p. 162; 1909, pp. 282-287). There are also in C. decip-
iens a few endoskeletal radials in connection with the pelvic girdle. The
lateral line was, on the trunk at least, partly surrounded by lime-bear-
ing tissue, but whether this tissue was bone or not it is not possible
to say. There are also in certain well-preserved specimens reliable evi-
dences of very delicate dermal tubercles behind the abdominal armour.
Contrary to what is generally maintained, I feel inclined to think
that the pelvic girdle, as well as all other endoskeletal elements that
are found preserved in Coccosteids, consists of true bone. A more
detailed microscopic examination of these elements has, however, so
far not been made, a fact which is much to be regretted.
'The remarks given here on the skeleton of the pelvic girdle and fins of
Coccosteus are based on observations made by the author in the Royal Scottish
Museum, Edinburgh and the British Museum, London. I fully agree with Wood-
ward (1891) in his interpretation of the specimens in the latter museum
(Dean 1909, p. 285).
180 Field Museum of Natural History — Geology, Vol. IV.
Some General Remarks on the Coccosteids
From what we know at present of the Coccosteids, it seems as if
they degenerated during the Devonian with regard to the degree of
ossification both in the exo- and endo- skeleton, a fact which to a cer-
tain degree was emphasized by Jaekel in 1906 (1906a p. 82). In
connection with this also a reduction of the pectoral spines took place.
THE HOMOSTEIDS
The Homosteids, represented by the genus Homosteus, are certainly
closely related to the Coccosteids.
The bones of the dermal cranial roof are the same as in the Coccos-
teids, but the dermal cranial roof as a whole differs from that in these
by the considerable lengthening of the occipital region and by the fact
that the orbital entrance is directed upwards and entirely situated in
it. It thus approaches in these respects that of the Phlyctenaspids on
the one hand and that of the Macropetalichthyids on the other.
The plate L is not found in Traquair's restoration (Traquair
1889) of the fish, a circumstance which probably indicates that it had
become entirely reduced. The anterior parts of the plates Px, A and R
lack both ornament and sensory canal grooves and must obviously have
been situated deep in the skin and are probably in an early stage of re-
duction. The other parts of the dermal cranial roof are provided with
a faint ornament, and this, in connection with the fact that certain
of the sensory canal grooves are lacking, while the remaining ones
are rather shallow and indistinct, seems to indicate that the dermal cra-
nial roof as a whole had commenced to sink down into the deeper layers
of the skin.
The skeleton of the cheek and the visceral skeleton are very im-
perfectly known and nothing can therefore be said of them here.
THE MYLOSTOMIDS
The Mylostomids, which probably are to be considered as an highly
specialized offshoot of the Coccosteids, differ from them mainly in the
characters of their dentition, the teeth, as we know, being in them
transformed to strong tritoral plates.
The dermal cranial roof is, at least in Mylostoma, said to be thin
and devoid of ornament and lacks most of the sensory canal grooves,
all of which conditions indicate that it was situated deep in the skin
and probably was in an initial stage of reduction. It may further be
mentioned that the plates homologous with L and Ps of Coccostens
are lacking and that the infraorbital plate (So) is narrow. The orbital
opening is not directly limited by any part of the dermal cranial roof.
Macropetalichthyids and other Arthrodires — Stensi5 181
In the lower jaw of Dinomylostotna, the Meckelian cartilage is ossi-
fied in the pars articularis (Woodward 1922, p. 34; Eastman 1906,
pp. 25-26). Eastman's view that the tritoral dentition of the Mylos-
tomids would be more primitive than that in the Coccosteids (East-
man 1906, p. 6; 1907; 1908a, p. 95; 1908b, p. 163) must, as is easily
understood, be incorrect (Cf. Woodward 1922, p. 35), for the Mylos-
tomids not only do not pertain to the latest Arthrodires (they are
from the upper Devonian), but they appear in most respects to be
more specialized than the Coccosteids from the lower Devonian. More-
over it would be very difficult to explain from Eastman's point of
view how the tritoral plates of the Mylostomids could be transformed
into teeth of the type found in the more primitive Coccosteids, as, for
instance, in Coccostcus.
THE PTYCTODONTIDS
The primordial neurocranium of Rhamphodus is said by Jaekel
(1906b, p. 183) to consist of calcified cartilage, and if I understand
Jaekel's account correctly this would also have been the case with
the visceral skeleton. I am not inclined to accept this view, however,
but rather think that we are in this case as in the Macropetalichthyids
concerned with true bone-tissue. Probably this bone-tissue, as in
Macropetalichthys, appeared chiefly or exclusively as thin perichondral
layers without evidence of sutures and distinct centres, conditions which
of course without microscopic investigation make the detection of its
true nature difficult.
I have more reason to suspect that the primordial skeleton of
Rhamphodus was, at least in part, actually ossified, since the primor-
dial neurocranium in a form pertaining to the family dealt with below
after the Ptyctodontids, as far as I could find by a macroscopic exami-
nation of it, was to a considerable extent ossified, although it is said
by Jaekel in a recent paper (1921, p. 217) to consist of calcified
cartilage (cf. the Jagorinids below)1.
Jaekel further mentions a "Schadeldach" in Rltamphodus, which,
if I understand him correctly, must mean that there is in this fish a
dermal cranial roof (Jaekel 1906b, p. 183).
Concerning the dentition, I have nothing new to add here. It must,
however, be emphasized that it is highly specialized. On account of
its agreements with that of the Chimaeroids, the Ptyctodontids have,
as is well known, been considered by several authors to be closely allied
to the Chimaeroids (Cf. Eastman 1908b, pp. 120-123).
'In another place in the same paper Jaekel is, however, obviously aware
that it was ossified (Jaekel 1921, p. 218).
1 82 Field Museum of Natural History — Geology, Vol. IV.
Rhamphodus has behind the head a dermal armour on the anterior
part of the trunk, an armour that, as is clearly shown by Jaekel, is of
the Arthrodiran type (Jaekel 1906b, Figs. 3, 5) although it has become
more reduced than is generally the case in the Arthrodires. In fact
this armour is represented merely by certain of the anterior plates
and the pectoral spine, which latter occupies its ordinary position. Its
agreements with a dermal shoulder girdle are obvious from Jaekel's
account, and from this it seems to be beyond question that the armour
of the trunk in the Arthrodires, at least in its anterior parts, really is
homologous with the dermal shoulder girdle of the Telostomi and
Tetrapods.
The trunk armour of the Ptyctodontids, as is seen from Jaekel's
figure of it, obviously formed part of the posterior boundary of the
branchial chamber and we thus have here an additional support for
my view that this chamber in Arthrodires did not extend backward
into the trunk. (Cf. Jaekel 1906b, Figs. 3-6; 1907, p. 184; the homo-
logue of the spinale also clearly understood from Jaekel's figures 3-6
in 1906a.)
THE JAGORINIDS
In a paper published in 192 1, Jaekel described under the name of
Jagorina pandora, certain remains of an upper Devonian fish, which
in this connection has a special interest. Through the courtesy of
Prof. Jaekel, I had, during a visit at Greifswald in 1922, the oppor-
tunity to examine the material so far available of this fish and the
account given below is therefore in part based on my personal ob-
servations.
Primordial Neurocranium
The primordial neurocranium is said by Jaekel in one part of his
paper, as already pointed out above, to consist of calcified cartilage,
while in another part it is said to have its outer and inner walls
feebly ossified. From the detailed account given by Jaekel and from
my own observations it is, however, fully evident to me that we are
concerned here with a primordial neurocranium that consisted chiefly
of cartilage, but which had the outside and inside (cerebral side) of
this cartilage provided with a thin, perichondral layer of bone exactly
as in Macropetalichthys rapheidolabis. There were, also, as in this
species, a labyrinth layer of perichondral bone and, at least, to a cer-
tain extent, canal layers too.
Unfortunately the single specimen that exhibits the primordial
neurocranium is preserved so that the primordial neurocranium is
Macropetalichthyids and other Arthrodires — Stensio 183
available for examination chiefly from the ventral side. The shape
from this side is shown by Fig. 26, which has been copied after Jaekel.
As is seen from this figure the ethmoidal region is very short and
broad, with a truncated, concave anterior margin and a large antero-
lateral^ projecting process on each side, a process {olf, Fig. 26),
which obviously is so situated and has such a shape, that it agrees
completely with the olfactory capsule of the Macropetalichthyids, and
is therefore undoubtedly to be interpreted as the olfactory capsule.
The orbits are very large and imperfectly bounded and the long orbito-
temporal region is consequently not distinctly marked off from the
labyrinth region, which probably also is rather long. The labyrinth
region is further characterized by the circumstance that it is broadest
Fig. 26. Jagorina pandora Jaekel
Primordial neurocranium in ventral view. After Jaekel (1921). XH- c°,
occipital condyle (paired); olf, nasal capsule; orb, orbit.
in its posterior parts, quite as the one in Dinichthys (Cf. Fig. 23 B).
The occipital region is short and its posterior end is provided with a
paired condyle for articulation with the vertebral column.
As a bottom for the orbit does not occur in the fish and the interor-
bital wall is fairly narrow in comparison with that in the Macropetali-
thyids, we must expect that the carotis interna passed forward along
the lateral edge of the ventral side of the primordial neurocranium and
not in the orbital bottom as in Macropetalichthys rapheidolabis. Con-
sequently it is probable that this artery when it turned medially to
ascend through the ventral wall of the primordial neurocranium to the
cavum cerebrale, entered a canal which opened in one of the pits
1 84 Field Museum of Natural History — Geology, Vol. IV.
situated on the ventral side of the anterior part of the primordial neuro-
cranium at about the transition between the orbitotemporal and ethmoid-
al regions. (As these pits had not been cleaned from matrix in their
deeper parts at the time I examined the specimen, I cannot, however,
say with certainty whether there were among them any that really
formed ventral openings for canals of this sort.)
The labyrinth cavity is well preserved and is evidently entirely
separated from the cavum cerebrale, quite as in M. rapheidolabis and
Selachians. The separating wall, which consisted of cartilage, was
lined by the internal bone layer on the cerebral surface and the
labyrinth bone layer on the labyrinth surface. A division for the
sacculus, divisions for semicircular canals and a dorsally ascending
canal for the ductus endolymphaticus are clearly distinguishable. The
canal for the ductus endolymphaticus opened on the dorsal side of the
primordial neurocranium and it seems probable also that in the living
fish it continued out to and opened on the external surface of the skin,
as it does in Epipetalichthys wildungensis, Selachians and Chimaeroids.
The canal for the n. acusticus issues from the cavum cerebrale very low
down, almost at the bottom and goes outward to the ventral parts of
the labyrinth cavity, probably almost as in M . rapheidolabis.
The shape of the caVum cerebrale is imperfectly known, but, as
far as can be judged at present, it appears to have resembled that in
M. rapheidolabis.
What function is to be ascribed to the wide canal that opens on, the
posterior side of the labyrinth region (Fig. 26) it is not possible to
decide at present, as its course inside the lateral wall is very imper-
fectly known. It appears most probable that it may have transmitted
the jugular vein, and perhaps, also, in its posterior parts, the vagus
nerve as well.
It is fully evident that the palatoquadrate cannot have been fused
with the primordial neurocranium, but must have been an independent
element.
Despite the fact that the primordial neurocranium of Jagorina
pandora is still in several points imperfectly known, we see, however,
that it is clearly of the Arthrodiran type (Cf. Fig. 23). It is therefore
beyond question that Jagorina pandora cannot be a sturgeon, as main-
tained by Jaekel, but that it must be referred to the Arthrodires.
Dermal Cranial Roof
The dermal cranial roof is, as far as is known, represented merely
by feebly developed isolated tubercules (called "Dornen" in the de-
scription given by Jaekel 1921, p. 219). The microscopic structure
Macropetalichthyids and other Arthrodires — Stensio 185
of these tubercles is not known and it is therefore impossible to say
whether they consist of bone or whether they are most comparable
to the dermal denticles of the sharks.
This fish thus has no real dermal cranial roof of bones united with
each other by sutures. The lack of a cranial roof of this kind must,
however, as far as we can judge, be due to reduction, since a real dermal
cranial roof occurs in all the other Arthrodires and is generally even
most complete in the forms from the lower Devonian.
Dermal Bones of the Cheek
No dermal structures belonging to the cheek have been observed
hitherto. „
Visceral Skeleton
Associated with the primordial neurocranium are found several
remains of the visceral skeleton, which was partly ossified. Among
these remains are surely parts of the palatoquad rates and the mandibles.
It is worthy of notice that the primordial neurocranium, as far as can
be seen, has no articulating surfaces for the anterior ends of the
palatoquadrates, and it therefore seems as if these had been suspended
by ligaments beneath the ethmoidal region, as in the Elasmobranchs.
Dentition
The dentition is, as Jaekel pointed out to me during my visit at
Greifswald, very interesting, as it consists of numerous, small, inde-
pendent1 Selachian-like teeth, thus teeth with several cusps. The shape
of these teeth is in fact of such a nature that they undoubtedly would
have been taken for shark teeth if they had been found detached from
the specimen, a fact which in this connection is of much importance,
as it forms a strong additional support for the view advanced by me
in this paper that the Arthrodires are really closely related to the Elas-
mobranchs. The microscopic structure of the teeth is not known.
It is worthy of notice in this connection that Woodward already in
192 1 shortly after Jaekel had published his description of the pri-
mordial neurocranium of Jagorina pandora, without any knowledge of
the character of the dentition, arrived at the conclusion that /. pandora
was a primitive shark (Woodward 1921, p. 33).
Sensory Canals
Nothing is known of the sensory canals of the head.
Abdominal Armour
Close behind the head there is a girdle of dermal plates that evi-
dently represents the shoulder girdle. It is not known in detail, but
*Not ankylosed with the jaws
1 86 Field Museum of Natural History — Geology, Vol. IV.
Jaekel says of it (1921, p. 218) that it is sturgeon-like, which may
mean very little, since he says the same of that in Rhamphodus (1906b,
pp. 183-185).
SOME GENERAL REMARKS ON THE NON-MACRO-
PETALICHTHYID ARTHRODIRES
From what has been set forth in the account given above the fol-
lowing ought to be obvious:
1. There is strong reason to believe that the primordial skeleton
of the non-Macropetalichthyid Arthrodires originally was rather well
ossified and that accordingly, a reduction of the degree of ossification
took place during the course of the Devonian period.
2. It is fully clear that the dermal skeleton of the non-Macropeta-
lichthyid Arthrodires was also during the Devonian period in process
of reduction. In certain, especially upper Devonian, forms the reduc-
tion has gone rather far and in Jagorina even so far that practically the
entire dermal skeleton of the head has been lost.
3. The reduction of the dermal skeleton in the non-Macropetalich-
thyid Arthrodires takes place in this manner : The bones sink deeper
into the skin, become thinner and lose their ornament and the external
sensory canal grooves. The sensory canals obviously must retain their
superficial relations to the skin and they become on this account sit-
uated totally externally of the bones.
4. From the facts so far known it is beyond doubt that the non-
Macropetalichthyid Arthrodires with regard to the degree of ossifica-
tion represent a degenerating series.
5. There are so many common points of agreement between the
non-Macropetalichthyid Arthrodires and the Macropetalichthyids that
we cannot doubt that they are closely related to each other and that
they must, as has generally been done hitherto, be referred to the
same group of fishes. •
6. From the structure of the primordial neurocranium, from the
development of the labyrinth cavity, from the character of the den-
tition in certain forms, and from the course and development of the
sensory canals, it is clear that the non-Macropetalichthyid Arthrodires,
like the Macropetalichthyids, must be closely related to the Elasmo-
branchs. In the case of Jagorina pandora this is even so clear, that
we should almost feel inclined to take it for a primitive Elasmobranch
if we did not know that it had a shoulder girdle of dermal bones.
Macropetalichthyids and other Arthrodires — Stensio 187
CONCLUDING REMARKS ON THE AFFINITIES OF THE
ARTHRODIRES
After having dealt in detail with the head of the Macropetalich-
thyids and also given a short account of the head in the other Arthro-
dires, I shall here briefly summarize the results obtained and add cer-
tain remarks of interest otherwise.
With the comparatively good knowledge we now possess of their
anatomy it is quite clear that the Arthrodires are true fishes and that
as recently pointed out by Woodward (1922, p. 35) they have noth-
ing to do with Ostracoderms, which as I have been able to make out,
are much more lowly organized agnathous vertebrates.
We have seen that the Arthrodires from the oldest divisions of the
Devonian generally have the dermal skeleton more complete and
stronger than those from the youngest division of the same formation,
and we have also found that the primordial skeleton is more or less
ossified in several of their representatives. Concerning the primordial
skeleton there have been advanced several facts which indicate that it
must have been rather completely ossified in all primitive Arthodires.
Accordingly we find that the Arthrodires like the Dipnoi, the Crossop-
terygii, the Palaoniscida, the Saurichthyida, the sturgeon fishes, the
Amiada and several other fishes, form a degeneration series with re-
gard to their degree of ossification.
In their general organization the Arthrodires have, as recently em-
phasized by Woodward (1922, p. 35), little in common with the Dipnoi
and Crossopterygii, and it is equally evident that they are not Actino-
ptcrycjii. From- the investigation made here they were, however, found
to be at the stage of the Elasmobranchs and in fact everything seems
to indicate that they are really closely related to these. Their Elasmo-
branchian characters are as follows :
I. The general characters of the primordial neurocranium, especially
the tendency to broadening of the ventral surface, partly at the ex-
pense of the lateral surfaces. (This is above all the case in the labyrinth
region, the sacculus having occupied about the same position in rela-
tion to the ventral surface as in Chlamydoselachus and most other
Selachians.) 2. The position and relations of the olfactory capsule.
3. The presence of a nasal fontanelle on the lower side of the olfactory
capsule (Macropctalichtliys) as in Chlamydoselachus and certain other
Selachians. 4. The presence of the cavum precerebrale (EpipetalicJi-
thys). 5. The general shape of the labyrinth, especially with regard
to the position of certain of its main parts, for instance, the utriculus.
6. The persistence of the ductus endolymphaticus (Macropetalichthys,
1 88 Field Museum of Natural History — Geology, Vol. IV.
Epipetalichthys, Jagorina) and the fact that there was probably, at
least in certain forms (Macropetalichthys) , a fossa endolymphatica on
the dorsal side of the primordial neurocranium beneath the dermal
bones. 7. The fact that the ductus endolymphaticus, at least in cer-
tain forms (Epipetalichthys), perforated the dermal cranial roof and
had an external opening situated about as in Chlamydoselachus. 8. The
general shape of the brain as far as this can be restored from the posi-
tion and course of the nerve canals through the cranial walls and from
the shape of the cavum cerebrale. 9. To a certain extent the develop-
ment of the blood vessels. 10. The fact that the palatoquadrate did
not articulate with the ethmoidal region but must have been suspended
beneath this merely by ligaments. II. The dentition, which in certain
forms, as Jagorina, consists of multicuspidated shark-like teeth. 12. The
course and development of the sensory canal system.
The opinion now advanced of the close relationship between the
Arthrodires and Elasmobranchs of course implies either that bone has
arisen independently in different groups of vertebrates or that it is a
very old sort of tissue that was present already among the most primi-
tive vertebrates and from these was bequeathed to the common ancestors
of the Arthrodires and Elasmobranchs. As has been pointed out, the
conditions in the Cephalaspids and Ostracoderms in general as well as
in the Acanthods and oldest Teleostomi indicate that the latter alter-
native ought to be the true one, and accordingly that the Elasmobranchs
as far as can be understood at present must have lost the bone-tissue.
In other words they too would represent a degenerating series with
regard to the degree of ossification. In full accordance with this view
is obviously the occurrence of large bone-like plates (or perhaps true
bone-plates) in the earliest known true forms of the Holocephali
(Myriacanthus and Chimeeropsis) and the fact that among the Cochlio-
dontids there is found a form (Menaspis) with evidence of armour on
the head and anterior part of the trunk (Cf. Woodward 1924) .*
Except by the presence of bone-tissue in their skeleton, a character
which, as we have found, is of very slight importance from a morpho-
logical point of view, the Arthodires seem, as far as we know, to
differ in no essential points from the Elasmobranchs. The different
shape of the vertebral column in them as compared with the recent
Elasmobranchs might perhaps be considered very insignificant, but if
we consider the fossil Elasmobranchs this difference becomes less ap-
*It is also of interest to note that Menaspis has paired lateral spines, which
perhaps are identical with the pectoral spines of Arthrodires. In fact it seems
not impossible that the Cochliodontids will reveal themselves as highly special-
ized Arthrodires (for Menaspis see Zittel's Grundzitge der Palaeontologie, 1923;
Dean 1904, and Jaekel 1891).
Macropetalichthyids and other Arthrodires — Stensio 189
parent, for in the Palaeozoic forms and certain earlier Mesozoic forms
of these the neural arches are almost like those in the Arthrodires (Cf.
Dean 1909, Clamydosclache; Fritsch 1895, Xenacanthus and Pleura-
canthus; Brown 1900 Hybodus; Koken 1907, Hybodus).
In addition to their general Elasmobranchian-like characters above
enumerated the Arthrodires have also certain special Holocephalian char-
acters the following of which deserve to be specially mentioned here.
1. The joint between the head and the vertebral column. 2. The
structure of the branchial apparatus, inasmuch as this is covered ex-
ternally by a sort of gill-cover. 3. The character of the dentition in
the more specialized forms. 4. The dorsal extension of the pelvic
girdle. 5. The general shape of the body. 6. Certain characters of the
arterial system.
Of these characters, No. 3 is quite certainly simply a parallelism and
it seems highly probable that this is the case with No. 5 too. Concern-
ing the remaining four it is difficult to say at present how much stress
ought to be laid on them, and it is consequently difficult to conclude
whether the Arthrodires among the Elasmobranchs were most closely
allied to the Holocephali or to the Selachii or whether they represent an
independent branch from the Elasmobranchian stem. Since, as we have
seen, they seem to have the pectoral fins much transformed or perhaps
represented merely by an anterior spine, it is not probable that they
could have given rise to the Holocephali. If it should appear that they
are most closely allied to these the Arthrodires would have descended
from the same primitive ancestors or, if we want to express it more
exactly, from the primitive forms of the Holocephalian branch of the
Elasmobranchian stem. Under the present conditions we can, however,
with assurance only say that they are to be considered as an offshoot of
the Elasmobranchian stem of fishes and that besides their Elasmo-
branchian characters they have retained a few primitive ones at the
same time that they by specialization have acquired certain new ones.
Their main primitive characters must, as far as we understand, be
the presence of bone-tissue in their skeleton and the occurrence of a
postero-medial branch for the infraorbital sensory groove on the der-
mal cranial roof.
As in another paper I shall give a detailed account of the relation-
ship between the Arthrodira and the Antiarcha, I shall here only men-
tion that, as far as I can see from the investigations of material in the
chief American and European Museums, as well as from material re-
cently obtained from Scaumenac Bay, the Antiarcha seem to be a highly
specialized group of fishes closely related to the Arthrodira.
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1907. Uber Pholidosteus n.g. Die Mundbildung n. die Korperform der Pla-
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191 1. Die Wirbelthiere. Berlin.
1919. Die Mundbildung der Placodermen. Berlin. Gesellsch. Naturforsch.
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1921. Palaeontologische Berichte. II. Schadelprobleme. Palaeontologische Zeit-
schrift, Vol. 3.
Johnston, J. B.
1898. Hind brain and cranial nerves of Acipenser. Anat. Anz., Bd. 28.
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Kaer, J.
1915. Upper Devonian fish remains from Ellesmere Land. Rep. of the Second
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191 6. Spitzbergens Devoniske Faunaer. i6de Skandinaviska Naturforskarmotet.
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1897. The structure and morphology of the oblongata in fishes. Journ. of
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1916. The cerebral ganglia and early nerves of Squalus acanthias. Journ.
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1909. Untersuchungen uber die vom N. trigeminus innervierte muskulatur der
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1908. The lateral line system in extinct Amphibia. Journ. Morph., Vol. 14.
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1889. The Palaeozoic fishes of North America. U. S. Geol. Surv., Monogr.,
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O'Donoghue, Ch. H.
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tem of the dogfish (Scyllium canicula). London. Zool. Soc, Proc.
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1882. On the structure and development of the skull in Sturgeons (Acipenscr
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EXPLANATION OF PLATES
All the photographs here shown of the specimen of Macropeta-
lichthys rapheidolabis in the possession of Field Museum of Natural
History at Chicago were made by the photographic department of that
Museum. The other specimens figured here were photographed by
myself in the photographic laboratory of the Palaeozoological Depart-
ment of the Royal State Museum at Stockholm.
In those cases in which no special statements as to the scale of the
figures are given the figures have been reproduced in natural size or
have been slightly diminished.
In Fig. 4, PI. XXIX and Fig. 2, PI. XXXI, the bone is brighter than
the stone ; in all the other figures the bone is black or, in any case,
darker than the stone. Sections through cavities and canals or exter-
nal openings of canals are generally shown in a very bright, sometimes
even an almost white tone. Fig. 1, PI. XXX is an exception to this
however, the sensory canals and their tubuh and the external opening
of the canal for the ductus endolymphaticus being black.
EXPLANATION OF PLATE XIX
Macropctalichthys rapheidolabis. The specimen in Field Museum of Nat-
ural History at Chicago (Museum No. P 1154) in, dorsal view. The dermal
bones of the cranial roof being weathered away with the exception of the sensory
canal ridges, the outlines of the primordial neurocranium are rather clearly ex-
hibited. The specimen lacks the posterior narrow division of the occipital region,
and the most anterior part of the ethmoidal region is imperfectly preserved. The
ridge for the hindmost part of the cephalic division of the lateral line has become
so abraded that the canals for nerve branches to it from the canal for the n.
linese lateralis are clearly seen.
cl, fine canal coming from below; the canal probably transmitted a vagus
branch, a fine branch from the n. linear lateralis and perhaps in addition some
vessel ; c.pp, sensory canal commissure, probably corresponding to the posterior
head line of pit organs in fishes in general; ifc, infraorbital sensory canal; Ic,
cephalic division of the lateral line; Idi-ldz, dorsally ascending canals for nerve
branches to the lateral line; the canals issue from the canal (n./) for the n.
lineae lateralis ; orb, orbital entrance ; par, pineal foramen ; soc, supraorbital sen-
sory canal.
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY, VOL. IV, PL. XIX.
"I**
c.jop
ld\
LI6KAKY
UNIVERSIIY Of ILLINOIS
URBANA
EXPLANATION OF PLATE XX
Macropetalichthys rapheidolabis. Same specimen as in the preceding plate
but with a postero-lateral part of the cranial roof removed to show in dorsal
view the labyrinth cavity, a part of the cranial cavity and certain canals. As
all these structures are lined by a thin, perichondral bone layer, which is black
from infiltration by bitumen, they consequently appear black in the figures ex-
cept when there are sections through them.
c.ef, canal perhaps traversed by the arteria efferens hyoidea on its way to
the radix aorta? (lateral dorsal aorta) ; c.hy, canal for the vena hyoidea (prob-
ably, in the external part, for the arteria efferens hyoidea too) ; c.pp, sensory
canal commissure, probably corresponding to the posterior head line of pit-organs
in fishes in general ; c.sem.ant, division of the labyrinth cavity for the canalis
semicircularis anterior; cv, cavum cerebrale cranii (with its lining bone mem-
brane) ; only a posterior part of it is seen; d.cnd, canal for the ductus endolym-
phaticus, ifc, infraorbital sensory canal ; in, two canals for nerve branches to
the infraorbital sensory canal. The direction of the two canals makes it fully
obvious that the nerve-branches transmitted by them must have come from a
prootic nerve, probably the r. oticus lateralis or its equivalent; /«, the canal
for the jugular vein; only the posterior part behind the sinus si (see PI. XXII),
shown; lab.cav, labyrinth cavity (partly exposed. Like the cavum cerebrale it
has its lining bone membrane preserved and is therefore black in the figure) :
Ic, cephalic division of the lateral line; Idi-lds, canals ascending from the canal
n.l. for the n. linese lateralis to the cephalic division of the lateral line; Is, lamella
of bone connecting canals transmitting lateralis branches for the innervation of the
posterior part of the internal and external bone layers with each other ; orb, orbital
entrance ; par, pineal foramen ; soc, supraorbital sensory canal ; v.lb, canal from
the labyrinth ; cavity to the dorsal part of the proximal portion of the vagus canal.
The canal probably transmitted a vein. On the right side the canal is partly seen
in section close to its posterior end; X™, the canal for the vagus and the vena
cerebralis posterior (proximal part).
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY, VOL. IV, PL. XX.
par
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UNIVERSUY Of ILLINOIS
URBANA
EXPLANATION OF PLATE XXI
Macropctalichthys raphcidolabis. Same specimen as in Pis. XIX and XX.
Primordial neurocranium in ventral view. Certain parts of the left side (right
in the figure) and of the anterior end removed.
c.car.ext, canal for the arteria carotis external (represented merely by a
groove, as its filling of stone and its external wall have been destroyed by
weathering) ; c.car.int, canal for the arteria carotis interna (represented to a
large extent by its filling of stone, as its external wall has been abraded). The
most anterior white spot marks the place at which the internal carotid curved
upward to the cavum cerebrale; c.com, canal for the arteria carotis communis
(its external wall and in part also its stone filling destroyed. In the anterior
part, in which the stone filling is lost, it appears as a groove) ; c.hy, canal for
the vena hyoidea (merely the proximal part completely preserved in the figure,
the distal part being, as is well seen, represented there by a groove) ; c.ophth.lat,
canal for the n. ophthalmicus lateralis (anterior part) ; c.paU the canal for the
r. palatinus facialis through the orbital floor (dorsal opening) ; era, canal for
the radix aortae (lateral dorsal aorta). The canal is imperfectly preserved,
being represented merely by a groove in the figure ; csem, division of the labyrinth
cavity for a semicircular canal, probably the c. semicircularis externas ; cv, cavum
cerebrale cranii (merely a small posterior part is shown) ; dtx, indication of a
canal, perhaps for the lateralis branch that accompanied the n. glossopharyngeus
from the cavum cerebrale; jgx, canal for the jugular vein from the orbit to the
mandibular vein (section) ; ju, canal, which through its anterior opening trans-
mitted the vena mandibularis (the anterior part not fully preserved. The
anterior opening was situated more laterally than in the figure) ; jih and jih, an-
terior and posterior opening of the canal ju; na, nasal aperture; nf, nasal fon-
tanelle ; n.L, canal for the n. linae lateralis ; olf, olfactory capsule ; rm, ophth. lat,
branch for a lateralis nerve from the canal for the n. ophthalmicus lateralis to the
supraorbital sensory canal ; si, sinus formed by the confluence of the canal ju and
the canal era (its ventral wall has been removed, so that its stone filling covered
by the lining membrane of bone is visible) : soc, supraorbital sensory canal (a
small anterior part) : s.ra, groove for the radix aortae (lateral dorsal aortae) ; Sx,
groove anterior of the external opening of the vagus canal ; VII, canal for the
n. facialis on the left side — right in the figure — the external part in section;
IX, canal for the n. glossopharyngeus (external opening) ; X™, external opening
of the canal for the vagus and the vena cerebralis posterior.
FIELD MUSEUM OF NATURAL HISTORY
olf
GEOLOGY, VOL. IV, PL. XXI.
c.ophth.lat
rm.ophth.la.t
-c.car.int
LIBRARY
cRSiTY Of ILLINOIS
URBANA
EXPLANATION OF PLATE XXII
Macropetalichthys rapheidolabis. Same specimen as in the three preceding
plates. Imperfect impression of the ventral surface of the primordial neuro-
cranium with certain lateral parts of this remaining in their original position on
the left side.
c.car.int, canal for the arteria carotis interna (the white opening denoted
is the place at which the artery turned upwards and ascended to the cavum
cerebrale) ; c.hy, canal for the venal hyoidea (on the right side is the external
opening shown as impression) ; c.cf, canal perhaps for the arteria efferens hyoidea
on its way to the radix aortae (lateral dorsal aorta) ; c.ophthJat, canal for the
n. ophthalmicus lateralis; c.ophth.sup.Vf, canal perhaps for the r. ophthalmicus
superficialis V through the antorbital process (merely the anterior opening seen
as impression in the figure) ; c.pah, canal for the r. palatinus facialis through
the orbital floor (merely the ventral opening into the internal carotid canal is
seen here) ; era, canal for the radix aortae (lateral dorsal aorta) ; ifc, infraor-
bital sensory canal ; in, two canals for nerve branches to the infraorbital sen-
sory canal. The direction of the canals makes it fully evident that the nerve
branches transmitted by them must have come from a prootic nerve, probably
the r. oticus lateralis or its equivalent; jg, canal for the vena jugular is from the
orbit to the mandibular vein (section) : ju, canal for the jugular vein pos-
terior of the sinus si; juu anterior opening of the canal ju. The opening was
traversed by the mandibular vein (impression) ; olf, olfactory capsule (impres-
sion of ventral side) ; si, sinus arisen by the confluence of the canals ju and
era; s.pal, groove for the r. palatinus facialis on the lower side of the primor-
dial' neurocranium (seen as impression here and therefore appearing as a ridge) ;
s.ra, groove for the radix aortae (lateral dorsal aorta). The groove is preserved
as an impression and has therefore the shape of a ridge; VII, canal for the
n. facialis (section) ; IX, canal for the n. glossopharyngeus (section close at the
external opening into the groove s.ra; Xvn, canal for the vagus and the vena
cerebralis posterior (abraded impression of the left external opening).
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY. VOL. IV, PL. XXII.
ophth.lat
jyr^j***sp
ophth
s r<x
LIBRARY
UNIVERSITY OF ILLINOIS
URBANA
191
EXPLANATION OF PLATE XXIII
Macropetalichthys rapheidolabis. Same specimen as in the preceding plates.
The primordial neurocranium in ventral view with certain parts removed on
the left side (right in the figure) to show the cavum cerebrale and the exits
of the canals for the cranial nerves from this.
afr, acustico-facialis recess; a.opt, division of the external opening of the
opticus canal traversed by the arteria optica; c.car.ext, canal for the arteria
carotis externa (as its filling of stone and ventral wall have been destroyed, it
appears as a groove in the figure) ; c.car.int, canal for the arteria carotis in-
terna. On the right side (left in the figure) the suborbital part is in almost the
entire extension represented by the filling of stone, as the ventral wall has been
destroyed. The foramen somewhat in front of the stone filling is the place
at which the artery curved upward to ascend to the cavum cerebrale. On the
left side (the right in the figure) is seen the ascending part of the same canal;
c.com, canal for the arteria carotis communis (in its anterior part, in which the
ventral wall and stone filling have been destroyed, it appears as a groove, in its
posterior part, in which merely the external wall is lacking, the filling of stone
is clearly seen) ; c.hy, canal for the vena hyoidea (the distal part destroyed and
represented in the figure merely as a groove) ; c.ophth.lat, canal for the n. oph-
thalmicus lateralis (merely an anterior part and a posterior part discernible) ;
c.ophth.prof, canal for the r. ophthalmicus profundus (proximal part) ; c.ophth.
sup.V, canal for the r. ophthalmicus superficialis trigemini to the orbit (entire
canal with the external opening) ; c.ophth. sup. Vf, canal probably for the
r. ophthalmicus superficialis trigemini through the preorbital process ; era, canal
for the radix aortae (lateral dorsal aorta). Its external wall and filling being
destroyed it appears merely as a groove; cv, cavum cerebrale cranii (with its
lining membrane of bone — the inner bone layer) ; dn, indication of a canal to
the dorsal side of the cranial roof, probably for a lateralis branch that accom-
panied the n. glossopharyngeus from the cavum cerebrale; jih, anterior opening
of the canal ju (as the lateral parts are broken off the actual anterior opening
of the canal ju was situated more laterally than in the figure. We are there
in fact concerned with a section through the part of the canal ju situated an-
terior of the sinus si) ; jih, position of the posterior opening of the canal ju
(the external wall and the filling of stone being destroyed, only a groove is
seen at this place in the figure) ; na, nasal aperture; nf, nasal fontanelle; n.l,
canal for the n. lineae lateralis ; olf, olfactory capsule ; rm.ophth.lat, fine canal for
a branch from the n. ophthalmicus lateralis to the supraorbital sensory canal;
si, sinus in the lateral wall arisen by confluence of the canals ju and era; soc,
supraorbital sensory canal (anterior part) ; s.ra, groove for the radix aortae
(lateral dorsal aorta) ; S*, groove leading from the vagus canal ; I, canal for
the tractus olfactorius; II, canal for the n. opticus (entire canal with external
opening) ; III, canal for the n. oculimotorius ; V, trigeminus recess ; Vi, i canal for
the r. maxillaris and r. mandibularis trigemini (section through the proximal
part) ; VII, canal for the n. facialis (proximal part in section) ; VIII, canal for
the n. acusticus ; IX, canal for the glossopharyngeus (proximal part) ; Xtd, canal
for the n. vagus and the vena cerebralis posterior.
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY, VOL. IV, PL. XXIII.
cophth.'ar
; rmophT'i I at
,-Soc
7,1
LIBRARY
UNIVERSITY OF ILLINOIS
URBANA
EXPLANATION OF PLATE XXIV
Macropetalichthys rapheidolabis. Same specimen as in the preceding plates.
Fig. I. Primordial neurocranium seen from the right side.
Fig. 2. A transversal section through the lateral part of the primordial neu-
rocranium, along the canal for the n. vagus and the vena cerebralis posterior. The
impression of the anterior side of this canal is shown in black. In one place a
part of its filling of stone remains (with white external end).
Fig. 3. Primordial neurocranium in posterior aspect. The place from which
the posterior narrow division of the occipital region issues is well shown. Further,
we can see at this place the posterior end of the middle division of the cavum
cerebrale and a section through the cavum cerebrale at the transition to the
posterior narrow division (white). The canal for the n. lineae lateralis is ex-
posed during a rather large part of its course, as the cranial wall has been
destroyed at this place.
c.hy, canal for the vena hyoidea (the septum between it and the anterior
opening of the canal ju, partly destroyed) ; c. 0 phth.su p. V, canal through the
preorbital process, probably for the r. ophthalmicus superficialis trigemini; c.pp,
sensory canal commissure, probably corresponding to the posterior head line of
pit organs in fishes in general; cv, cavum cerebrale cranii (a section of it at
the transition to the posterior narrow division is shown in white) ; dix. indication
of a canal to the dorsal side of the primordial neurocranium. The canal pro-
bably transmitted a lateralis branch that might have accompanied the n. glosso-
pharyngeus from the cavum cerebrale; dx, dorsal branch from the vagus canal
for a lateralis nerve; dxn, and dn>, branches of the canal ds, the former of
which lodged the nerve to the antero-laterally running part of the cephalic di-
vision of the lateral line, while the latter lodged the nerve to the sensory canal
commissure c.pp; ifc, infraorbital sensory canal ; jui, anterior opening of the
canal ju (the septum between it and the external part of the canal c.hy, partly
destroyed); Ic, cephalic division of the lateral line; ldi-lde, canals for branches
from the n. lineas lateralis to the most posterior part of the cephalic division of
the lateral line ; na, nasal aperture ; nf, nasal f ontanelle ; n.l, canal for the lineae
lateralis (exposed in the distal part owing to the incomplete state of preservation of
the posterior cranial wall ; orb, orbit ; v. lb, canal from the postero-dorso-medial
part of the labyrinth cavity to the proximal dorsal part of the vagus canal. The
canal probably transmitted a vein (section) ; vy, opening of a canal of doubtful
importance (perhaps for some vessel) ; VII, canal for the n. facialis; XV, canal
for the n. glossopharyngeus ; Xvn, canal for the n. vagus and the vena cerebralis
posterior (the two divisions are clearly shown).
LIBRARY
UNIVERSITY OF ILLINOIS
URBANA
EXPLANATION OF PLATE XXV
Macropetalichthys rapheidolabis. Same specimen as in the preceding plates.
Fig. i. Primordial neurocranium from the left side with certain lateral parts
removed to show parts of the cavum cerebrale.
Fig. 2. Primordial neurocranium in the same aspect as in the preceding figure
but with still another lateral part removed to show the interorbital wall.
Fig. 3. The surface orp in fig 1, seen from behind. This surface is the
impression of the posterior surface of the orbit.
afr, acustico-facialis recess ; a.opt, division of the external opening of the
opticus canal for the arteria optica ; c.car.int, canal for the arteria carotis in-
terna (the ascending portion) ; c.ophth.lat, canal for the n. ophthalmicus lateral-
is (proximal part) ; c.ophth.prof, canal for the r. ophthalmicus profundus to the
orbit (proximal part) ; c.ophth.sup.V, canal for the r. ophthalmicus superficialis
trigemini to the orbit; cv, cavum cerebrale (middle division, which is covered
by its lining layer of bone — the inner bone layer) ; jg, canal for the jugular vein
from the orbit to the confluence with the mandibular vein ; na, nasal aperture ;
orb, orbit. In fig. 1 it is filled with stone, in fig. 2 this filling has been re-
moved so that its medial wall is seen partly with its external surface preserved
(black, as it is covered by the external bone layer) and partly in section (bright,
with nerve canals in it) ; orp, posterior surface of the orbit preserved as an im-
pression (to a large extent with the external bone layer adherent to it and there-
fore black in the figure) ; v.lb, canal from the postero-dorso-medial part of the
labyrinth cavity to the proximal dorsal part of the vagus canal. The canal
probably transmitted a vein (here is merely seen a section through the posterior
part) ; Vt, Vi, two canals leading from the cavum cerebrale to the labyrinth cavity,
probably for vessels ; II, canal for the n. opticus (external opening) ; III, canal
for the n. oculomotorius ; V, trigeminus recess ; Vs, $, canal for the r. maxillaris
trigemini and the r. mandibularis trigemini (section through the proximal part) ;
VII, facialis canal (several parts of it are seen). Figs. I and 2 show that it is
crossed on the dorsal side by the canal jg; VIII, canal for the n. acusticus; IX,
canal for the n. glossopharyngeus (proximal part) ; Xvn, canal for the n. vagus
and the v. cerebralis posterior.
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EXPLANATION OF PLATE XXVI
Macropetalichthys rapheidolabis. Same specimen as in the preceding plates.
Figs. 1, 2. The cavum cerebrale seen from the left side (partly in section).
Fig. 3. The labyrinth cavity seen from the medial side with the wall separat-
ing it from the orbit (the posterior surface of the orbit denoted by orp) and
the vagus canal (Xvn,) shown in their positions and with their real thickness.
Fig. 4. The labyrinth cavity shown from the lateral side (imperfectly ex-
posed).
afr, acustico-f acialis recess ; c.ophth.prof, canal for the r. ophthalmicus pro-
fundus to the orbit (proximal part) ; c.ophth.sup.V, canal for the r. ophthalmi-
cus superficialis trigemini to the orbit proximal part) ; csem, divisions of the
labyrinth cavity for a semicircular canal, perhaps the c. semicircularis externas ;
csem. ant, division of the labyrinth cavity for the canalis semicircularis anterior ;
cv, cavum cerebrale (with its lining bone layer — the inner bone layer — where it is
shown in black ; sections through it are shown in white) ; d.end, canal for the
ductus endolymphaticus ; ep, diverticle from the posterior part of the roof of
the anterior division of the cavum cerebrale (not seen in its entire length in the
figures) ; orp, posterior surface of the orbit; par, pineal canal; ru, division of
the labyrinth cavity occupied by the recessus utriculi; utr, division of the laby-
rinth cavity occupied by the utriculus ; sac, division of the labyrinth recess
occupied by the sacculus (partly seen) ; v. lb, canal from the labyrinth cavity to
the dorsal proximal part of the vagus canal (probably for a vein) ; v*, Vxx, canals
probably for vessels ; Vi, Vz, canals probably for vessels from the cavum cerebrale
to the labyrinth cavity ; I, canal for the tractus olfactorius ; III, canal for the
n. oculomotorius (proximal part) ; V, trigeminus recess ; V», », canal for the r.
maxillaris trigemini and the r. mandibulars trigemini (proximal part in section) ;
VII, canal for the n. facialis (in fig. 3 a considerable part of it is seen in the wall
between the labyrinth cavity and the orbit, as the ventral part of this wall has
been removed) ; VIII, canal for the n. acusticus ; IX, canal for the n. glosso-
pharyngeus; X™, canal for the n. vagus and the vena cerebralis posterior (in
figs. 1, 2, section through its proximal part, in figs. 3, 4, impression of its anterior
side. In fig. 4 also, a part of its stone filling is seen, the anterior end of which
is white).
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EXPLANATION OF PLATE XXVII
Macropetalichthys rapheidolabis. Specimen 710 F in the American Museum
of Nat. Hist., New York. About 34 of the nat. size.
The specimen exhibits certain basal parts of the primordial neurocranium.
cv, cavum cerebrale cranii, the posterior narrow division (in section at the
place denoted. Behind this place covered by the internal bone layer il) ; el, the
external bone layer (the part of it on the ventral surface of the occipital
labyrinth and orbitotemporal regions and a part of it on the dorsal side of the
occipital region preserved. The latter part has, however, been partly removed
by preparation) ; il, internal bone layer (surrounding the posterior narrow divi-
sion of the cavum cerebrale) ; lab.cav, labyrinth cavity (in horizontal section) ;
Idi, canal for a nerve branch from the n. linese lateralis to the posterior part of
the cephalic division of the lateral line; n.l, canal for the n. linese lateralis (its
origin from the vagus canal clearly seen here) ; orb, orbit (its floor partly pre-
served on the left side) ; I, canal for the tractus olf actorius ; IX ?, perhaps the
canal for the n. glossopharyngeus ; X-m, canal for the n. vagus and the vena cere-
bralis posterior.
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY, VOL. IV, PL. XXVII.
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EXPLANATION OF PLATE XXVIII
Figs. I, 2. Epipetalichthys ivildungensis. Specimen belonging to the Geologi-
cal Institution of the University of Greifswald, Germany. Fig. i shows the
parts preserved of the ethmoidal region of the primordial neurocranium from the
right side; fig. 2 shows the parts preserved of the occipital and labyrinth regions
in ventral aspect.
Figs. 3, 4. Macropetalichthys rapheidolabis. Specimen 4445 G of the Ameri-
can Museum of Nat. Hist., New York. Fig. 3 is in posterior, fig. 4 in ventral
view. The specimen displays the posterior narrow division of the occipital region
of a rather large animal.
Fig. 5. Macropetalichthys rapheidolabis. Specimen 280 G of the American
Museum of Nat. Hist. New York. Portion of the posterior narrow division of
the occipital region in ventral view. The ventral parts of the division and the
cavum cerebrale have been removed so that merely certain dorsal parts remain.
What is lettered el is here the anterior part of the external bone layer of the
dorsal side of the division. The bone piece lettered it is a posterior part of the
inner bone layer on the dorsal side of the cavum cerebrale.
ch, space for the notochord (just at the anterior end) ; ch.w, ridge in the
anterior part of the haemal groove caused by the notochord; c.rai, c.rai, anterior
and posterior opening of a canal for the radix aortae (lateral dorsal aorta). The
canal corresponds to a posterior part of the groove s.ra in M. rapheidolabis ; cr.ol,
crista occipitalis lateralis ; cr.sp, craniospinal process (impression of its anterior
surface; fragments in fig. 3); el, external bone layer; hcem, haemal groove; il,
internal bone layer; na, nasal aperture; orb, orbital entrance; pr, postero-laterally
projecting process of the anterior broad division of the occipital region.
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.
EXPLANATION OF PLATE XXIX
Figs, i, 2. Macropetalichthys agassisif Specimen in the possession of Mr.
S. Junkermann of Bielefeld, Germany. Fig. i shows much crushed posterior
parts of the primordial neurocranium in ventral view ; fig. 2 a posterior part of
the dermal cranial roof with a number of sensory canal pores.
Figs. 3, 4. Epipetalichthys wildungensis. Specimen belonging to the Geo-
logical Institution of the University of Greifswald, Germany. Same specimen
as in figs. I, 2, PI. XXVIII. Fig. 3 shows the part preserved of the ethmoidal
region in ventral view, fig. 4 the ornament on a part of the dermal cranial roof
(impression with the bone tissue of the tubercles adhering to the stone and
appearing brighter than this).
c.pp, sensory canal commissure probably corresponding to the posterior head
line of pit organs in fishes in general; cr.od, crista occipitalis dorsalis (impres-
sion) ; d.end, dorsal opening of the canal for the ductus endolymphaticus ; Ic,
cephalic division of the lateral line; na, nasal aperture; olf, olfactory capsule;
pr, postero-laterally projecting process of the broad anterior division of the
occipital region; s.pal, groove for the r. palatinus facialis.
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EXPLANATION OF PLATE XXX
Fig. i. Epipetalichthys wildungensis. Specimen belonging to the Geological
Institution of the University of Greifswald, Germany. Same specimen as in
figs, i, 2, PI. XXVIII and figs. 3, 4 in PI. XXIX. Head in dorsal view. The
head lacks a large posterior part. Anteriorly it shows the cavum precerebrale
and the olfactory capsule. The approximate position of the sutures between the
bones of the dermal cranial roof is indicated with black lines.
Fig. 2. Macropetalichthys rapheidolabis. Specimen 280 G of the American
Museum of Nat. Hist., New York. The posterior end of the occipital region in
posterior views. The cranio-spinal process {cr.sp) is well shown, but is repre-
sented only by the impression of its anterior side.
Mi, Mt, Li, L2, L», Pi, P2, S, dermal bones of the cranial roof ; Mi imper-
iect anteriorly; cr.od, crista occipitalis dorsalis (its posterior continuation on
the anterior surface of the cranio-spinal process) ; cr.sp, cranio-spinal process
(represented merely by the impression of its anterior surface) ; d.end, the canal
for the ductus endolymphaticus (dorsal opening) ; ifc, infraorbital sensory canal
(its position indicated by its pores) ; il, internal bone layer; Ic, cephalic division
of the lateral line (its posterior part indicated by pores; its anterior part does
not open outwards); olf, olfactory capsule; orb, orbital entrance; pfg, cavum
precerebrale; soc, supraorbital sensory canal (exposed to a large extent by
weathering) st.com, cross-commissural sensory canal, probably representing the
true supratemporal commissure of fishes in general.
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY, VOL. IV, PL. XXX.
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9dl
EXPLANATION OF PLATE XXXI
Fig. I. Macropetalichthys rapheidolabis. Microscopic section through the
external bone layer of the primordial neurocranium. Magnification 700/1.
Fig. 2. Epipetalichthys wildungensis. Specimen belonging to the Geological
Institution of the University of Greifswald, Germany. Part of a section through
the anterior part of the posterior narrow division of the occipital region. Note
the trabecles on the inside of the external bone layer {el) and on the outside of
the notochordal layer (ch.l). Magnification about 30/1.
ch, space for the notochord (partly seen); ch.l, bone layer surrounding the
notochordal space; cs, cell space; el, external bone layer; ham, haemal groove.
FIELD MUSEUM OF NATURAL HISTORY.
GEOLOGY, VOL. IV, PL. XXXI.
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