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L I E. RAFLY

OF THE

UN IVERSITY

or ILLINOIS

GEOLOGICAL SERIES

OF
FIELD MUSEUM OF NATURAL HISTORY

Volume VI Chicago, October 31, 1936 No. 15

THE INTERNAL STRUCTURE OF THE EAR
IN SOME NOTOUNGULATES

By Bryan Patterson

Assistant Curator of Fossil Mammai^

Results of the First Marshall Field Paleontological Expedition
TO Argentina and Bolivia, 1922-24

The structure of the notoungulate auditory region has attracted
the attention of most investigators of the order, and several accounts
of the external structure have been written. The internal structure
of the bulla, however, has not received the same attention. Roth
(1903) published some sections, and various statements to the effect
that the bulla of some forms is hollow, or probably hollow, and that
the epitympanic sinus is in communication with the bulla may be
found in the literature, but no general account is available. Simpson
has recently published (1936) a very detailed description of the
cranial structure of the Casamayor genus Oldfieldthomasia, which
was studied by means of serial sections. Through his kindness, I
was able to see a manuscript copy after work on this paper had
been nearly completed. Simpson's paper covers a larger field than
the present contribution, but where the two studies are on common
ground our interpretations are in close agreement.

I was attracted to this subject by the accidental breakage of
a Hegetotherium skull which revealed the presence of a septum in
the bulla. From this beginning a study was made of representative
genera from such families as were available in the Field Museum
collections, in an endeavor to determine whether the middle ear
structure threw any light on problems of taxonomy and phylogeny.
The following contribution is by no means complete, since several
families are not represented, but it is hoped that it may serve as a
working basis. The majority of the specimens sectioned are from
the Santa Cruz beds. Material from this horizon is particularly
suitable for detailed investigations, due to the excellent preservation
of the bone and the softness of the matrix.

No. 367 199

Nstura] Ei«ttry Harvey

200 Field Museum of Natural History — Geology, Vol. VI

I wish to express my sincere thanks to Mr. Elmer S. Riggs for
permission to section the specimens in the Field Museum collections,
and to Dr. Walter Granger for similar permission in regard to the
American Museum specimen of Pseudotypotherium. The drawings
are the work of Mr. Carl F. Gronemann, Staff Illustrator, Field
Museum. I am indebted to Mr. James H. Quinn for his skillful
preparation of the Homalodotherium specimen.

Suborder Typotheria Zittel

Hegetotheriidae Ameghino

Hegetotherium Ameghino (fig. 44).

The description is based mainly on F.M. No. P13194, referred
to H. mirdbile Ameghino. The specimen was collected by Mr. Riggs
in the Santa Cruz beds on the Atlantic coast some twenty miles
south of the Rio Coyle inlet, Santa Cruz, Argentina.

The bulla is partially divided into two cavities by a vertical
antero-posteriorly aligned septum. This septum is low and attached
anteriorly, ventrally, and posteriorly to the walls of the bulla.
Beneath the promontorium it is free, leaving a rather large opening
between the two chambers. From its relations, this septum could
well be regarded as a true septum bullae formed by union of ecto-
and entotympanics (see p. 222 for a discussion of this question).
The medial chamber of the bulla— the hypotympanic sinus — is smaller
and much shallower dorso-ventrally than the lateral — the tympanic
cavity. It is hollow, oval in horizontal section, and somewhat
deeper than wide. The anterior extremity of its wall is thickened.
The tympanic cavity is much deeper than wide, rounded laterally,
and tapering anteriorly. The anterior half of the cavity, at least as
far back as the tympanic aperture of the facial canal, is roofed by the
tympanic. The Eustachian tube is therefore entirely surrounded by
this element, except possibly at its extreme outer margin where it
seems to open between tympanic and alisphenoid. The tube runs
antero-internally and is situated lateral to the anterior part of the
septum; it opens externally a little below and behind the foramen
ovale. The wall of the bulla is closed below the tube, and there is
no indication of a styliform process. The recessus epitympanicus is
of medium size, and the pneumatic foramen is relatively much
larger than that of Nesodon. There is a small recess anterior to this
foramen. The sulcus tympanicus is large and well defined, and the
crista tympanica prominent. The recessus meatus is shallow.

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202 Field Museum of Natural History — Geology, Vol. VI

The promontorium in this and the other typotheres studied is large. It
forms a large part of the medial wall of the tympanic cavity and a
small part of the roof of the hypotympanic sinus. The fenestra
cochlearis is round and faces posteriorly; the fenestra vestibuli is oval
with the long axis aligned antero-posteriorly, and faces laterally.

The details of the carotid circulation are somewhat uncertain
in the forms described here. In Hegetotherium, Sinclair (1909, p.
74) identified as the aperture of the carotid canal a foramen which
notches the basioccipital near the mid-line of the bulla. In the
specimen under consideration, this foramen leads anteriorly into
a canal which runs between the medial wall of the bulla, the ventro-
medial margin of the periotic, and the basioccipital. This canal
opens into the cranial cavity at the anterior extremity of the periotic.
This internal orifice occupies approximately the same position on the
inside of the skull as the sectioned canal identified as the carotid by
Simpson (1933, fig. 1) occupies on the brain cast. The evidence in
favor of regarding this foramen and canal as being for the internal
carotid seems to be sound. In P13194 I have detected a small groove
in the posterior external wall of the bulla between the tympanic and
periotic. This apparently continues medially beyond the foramen
lacerum posterius (broken away in the specimen) into a minute
foramen in the periotic situated just above the groove for the basi-
occipital between the periotic and the medial wall of the bulla. Lat-
erally this groove continues into the tympanic cavity between the
squamosal and tympanic, ending at the internal orifice of the sty-
lomastoid foramen. At first I thought that this groove might have
lodged a minute stapedial artery (pterygopalatine portion of the
internal carotid) which would therefore have branched off from
the internal carotid outside the bulla. However, after finding the
apparent medial extension of this groove to the periotic and noting
its lateral connection with the stylomastoid foramen, I am in-
clined to believe that it more probably lodged some branch of the
stylomastoid artery (in man a branch of the posterior auricular).
Two small unidentified foramina, probably vascular, that leave the
tympanic cavity, have been noted. One of these pierces the tympanic
in front of the promontorium and the other leaves the cavity opposite
the apertura tympanici canalis facialis.

The venous sinus in the squamosal is well shown on the figured
specimen anterior to the epitympanic sinus. It is partially enclosed
medially by the lateral face of the periotic. Essentially the same
foramina open into it as in Adinotherium (p. 214).

1

Ear Structure in Notoungulates 203

The course of the facial nerve (VII) is very clear in this specimen
and appears to be similar to that described by Simpson in Oldfield-
thomasia. The canalis facialis opens internally into the dorsal
portion of the internal auditory meatus directly above the orifice
for the auditory nerve (VIII), from which it is separated by a
low horizontal septum. Within the periotic, the canal is joined by
the hiatus facialis for the great superficial petrosal nerve, which
enters the bone on its anterior face near the junction with the
tympanic. The facial canal opens into the tympanic cavity a
little above and anterior to the fenestra vestibuli. The orifice is
bounded dorsally by the tympanic and ventrally by the periotic.
From the aperture a sulcus facialis runs backward and slightly
outward to the internal orifice of the stylomastoid foramen. The
latter is situated between the tympanic and squamosal above the
posterior "leg" of the sulcus tympanicus. The external opening of this
foramen is the same as in other notoungulates previously described
(Patterson, 1932). The canal of Huguier for the chorda tympani
begins internally between the tympanic and squamosal above the
anterior "leg" of the sulcus tympanicus. It opens externally at the
posterior extremity of the fissura Glaseri. The position of the
external orifice is constant in all notoungulates that I have examined.

Pachyrukhos Ameghino (fig. 45).

The following account is based on P.M. No. P12994 collected
by Mr. J. B. Abbott in the Santa Cruz beds at Killik Aike Norte,
Rio Gallegos, Santa Ciniz. It is referred to P. moyani Ameghino.

« hypt. sin.

car. f

flp.^^

yO.occ. pr

t-ym.

I
tym. cav.

Fig. 45. Pachyrukhos moyani Ameghino. F.M. No. P12991. Antero-ventro-lateral view of
auditory region of left side. Dotted areas represent sectioned bone. For abbreviations see page 227.
X 2/1.

204 Field Museum of Natural History — Geology, Vol. VI

The bulla differs in shape and proportions from that of Hege-
totherium, being much deeper dorso-ventrally and having a somewhat
greater transverse, than antero-posterior, diameter. The auditory
meatus is longer and inclined more dorsally. These departures from
the more generalized type exemplified by Hegetotherium are, of
course, accompanied by changes in internal proportions. The
hypotympanic sinus is smaller than the tympanic cavity and is
more antero-internal in position, the septum is aligned antero-
externally postero-internally, and the tympanic cavity is much
deeper and more dorsal in position than the hypotympanic sinus.
Posteriorly, it extends back above the paroccipital process.

Apart from proportions, however, the middle ear structure of
the two genera is essentially similar. The carotid circulation appears
to be the same in each; both have a prominent crista tympanica,
and a rather low vertical septum. Neither has a styliform process.
The most noticeable distinction in Pachyrukhos is the secondarily
thickened, cellular anterior and medial walls of the hypotympanic
sinus. In this character the genus parallels Protypotherium and the
typotherids, but the cellulae are more circular and have much thicker
walls.

Interatheriidae Ameghino

Interatherium Ameghino (fig. 46).

F.M. No. P13057, referred to /. robustum (Ameghino) and
collected by Mr. G. F. Sternberg in the Santa Cruz beds at Killik
Aike Norte, Rio Gallegos, Santa Cruz, clearly reveals most of the
middle ear structure.

The bulla is approximately quadrangular in horizontal section.
The hypotympanic sinus is hollow and considerably smaller than the
tympanic cavity. A remarkable feature, in comparison with the
hegetotherids, is the lack of a septum partially separating the two
chambers. However, small vertical ridges which may be vestiges of
this structure are present medial to the Eustachian tube and the
posterior carotid foramen. These ridges occupy the same positions as
the anterior and posterior extremities of the septa of Hegetotherium
and Pachyrukhos. This might indicate that the septum had been
present in the early members of the family but became reduced in
the course of later evolution.

The tympanic cavity is broader and much deeper than the hypo-
tympanic sinus; it is roughly rectangular with the greatest diameter
in the antero-posterior direction. The Eustachian tube extends

Ear Structure in Notoungulates

205

antero-internally and is entirely surrounded by the tympanic. It
opens externally below a shelf, formed by the tympanic, which
projects above it for a short distance. The anterior wall of the
hypotympanic sinus is vertical and meets this shelf at a right angle,
an arrangement peculiar to this genus. There is no styliform
process but, to judge from Protypotherium, I am inclined to regard
the lack of it as secondary. Above and on the medial side of the

£.1^

f.ov.

prom.

~fe.n.coch.

p.occ. pr.

I f:/ p.-/-p.c./:

Fig. 46. I lUeralherium robiislum (Ameghino). F.M. No. P13057. Ventral view of auditory region
of left side. Dotted areas represent sectioned bone. For abbreviations see page 227. X 2/1.

internal opening of the Eustachian tube there is a recess leading
antero-internally. In Protypotherium two foramina lead from this
recess; it is probable therefore that the same condition prevails in
this genus. The recessus epitympanicus is similar to that of Hege-
totherium, and the pneumatic foramen, as in that genus, is approxi-
mately half as large as the internal opening of the external auditory
meatus. The sulcus tympanicus is larger and deeper than in the
hegetotherids, but the crista tympanica on the other hand is much
less prominent. The recessus meatus, as in all the forms herein
described, is very shallow.

The carotid circulation in this, and in the remaining forms
described, differs decidedly from the arrangement seen in the Hegeto-
theriidae in that the internal carotid traversed the bulla instead
of passing forward medial to and outside it. In Interatherium the

206 Field Museum of Natural History — Geology, Vol. VI

posterior carotid foramen is small and situated on the lateral side of
the foramen lacerum posterius,^ It opens into the tympanic cavity
lateral to and above the posterior vertical ridge (remnant of the
septum?). I have not detected any arterial foramen leaving the
cavity but it is very probable that the recess above the Eustachian
tube ends in a foramen or foramina (see below under Protypotheriam).
The venous foramina and sinus, and the courses of the facial
nerve and chorda tympani offer no peculiarities.

Protypotherium Ameghino (fig. 47).

The figured specimen, F.M. No. P13002, was collected by Mr.
Sternberg in the Santa Cruz beds at Killik Aike Norte, Rio Gallegos,
Santa Cruz. It is referred to P. australe Ameghino.

C.Tf

-pg/. f

^ty. ptr

\ \ or. tym.

n ; \ » /

I . \r<z c. e.pi-.
-f^'ov. ^-f^

Fig. 47. Protypotherium australe Ameghino. F.M. No. P13002. Ventral view of auditory region
of left side. Three elements may be seen participating in the base of the paroccipital process, the
exoccipital forming the major part, the tympanic anteriorly, and the posterior adventitious bone
laterally. Dotted areas represent sectioned bone. For abbreviations see page 227. X 2/1.

This genus agrees with Interatherium in nearly all points of
structure. The following distinctions have been noted : The posterior
vertical ridge is lacking. The external opening of the Eustachian
tube is not widely open below, nor is it provided with a shelf-like roof
above. The reason for this is that the anterior wall of the tympanic

'In an earlier description of the interatherid Plagiarthrus {=Argyrohyrax)
I incorrectly figured the posterior carotid foramen as being internal to the foramen
lacerum posterius (1932, fig. 5a).

Ear Structure in Notoungulates 207

cavity sends a styliform extension medially, and a little anteriorly
and ventrally to underlie the anterior extremity of the hypotympanic
sinus. It is demarcated from the ventral wall of the sinus by a
groove. The sinus continues beyond it in an antero-internal
direction. Morphologically, this extension seems to be a styliform
process, but it is very different from the freely projecting processes
characteristic of the Toxodonta and Homalodotherium. The medial
wall of the hypotympanic sinus is cancellous and secondarily ex-
panded in P. australe. It extends very nearly to the lateral
extremity of the sinus. The cancelli are very fine and differ con-
siderably from the thicker and more regular filling of the expanded
medial wall in Pachyrukhos.^ The cancellous portion is shallow
dorso-ventrally.

The posterior carotid foramen is in the same position as in
Interatherium. At the anterior end of a small recess above and
medial to the Eustachian tube, homologous with the larger recess
in Interatherium, are the tympanic apertures of two small foramina.
The medial foramen of this pair is situated at the anterior end of the
promontorium between the periotic and tympanic, and leads antero-
internally to the cranial cavity. Its cranial orifice is situated above
and behind the foramen ovale and is bounded medially by the peri-
otic, laterally by the tympanic and alisphenoid. I regard this as
the anterior carotid foramen. The smaller lateral foramen tunnels
the tympanic in an anterior direction, emerging externally above the
orifice of the Eustachian tube and below the foramen ovale. This
foramen is almost certainly vascular and possibly may have trans-
mitted the stapedial artery. An equally possible alternative, how-
ever, is that the arteria promontorii was suppressed, the medial
(anterior carotid) foramen transmitting the stapedial artery and the
lateral its ramus inferior. The position and courses of these fora-
mina differ somewhat from the foramen regarded as anterior carotid
in Oldjieldthomasia by Simpson (1936, pp. 16-17). Complete uni-
formity can hardly be expected, however, among all forms in which
the carotid traversed the bulla. The vagina processus hyoidei
terminates in a minute foramen, probably vascular, which pierces
the tympanic in an antero-dorsal direction. I have not been able to
trace it completely.

1 This character is not common to all species, however. In a skull referred to
P. attenuatum Ameghino, F.M. No. P13050, the medial wall of the sinus is neither
expanded nor cancellous, and the medial portion of the sinus is deeper than in
P13002.

208 Field Museum of Natural History — Geology, Vol. VI

Typotheriidae Lydekker
Pseudotypotherium Ameghino (fig. 48).

The description is taken from A.M. No. 14509 from the Monte
Hermoso beds at Monte Hermoso, Buenos Aires. The specimen
is identified as P. pseudopachygnathum (Ameghino).

In the external characters of the auditory region the typotherids
bear some resemblance to the earlier toxodontids. The two families
have in common the high porus acusticus externus, the prominent
crista meati with its descending spine, and the lip from the crista
which forms the ventral border of the porus. The tympanic cavity

hypt.sin.

cr.m.

can. Hu^.
fen. vest.

'prom.

Fig. 48. Pseudotypotherium pseudopachygnathum (Ameghino). A.M. No. 14509. Ventro-lateral
view of auditory region of right side (crista meati restored from left side). For abbreviations see
page 227. X 3/2.

is situated relatively high on the side of the cranium, the greater
part of it dorsal to the hypotympanic sinus, and the Eustachian
tube is deep dorso-ventrally in each. These purely convergent
resemblances, possibly conditioned in part by the high porus acus-
ticus externus in each family, end here, however. Structurally, the
typotherid middle ear is a derivative of the interatherid type, such

Ear Structure in Notoungulates 209

as that seen in Protypotherium australe, a fact of considerable taxo-
nomic and phylogenetic interest (see p. 224).

The hypotympanic sinus is ventral in position, its lateral portion
being below part of the tympanic cavity. It is rather shallow dorso-
ventrally at its medial extremity. The medial wall seems to have
expanded laterally, as in Protypotherium australe, until it has filled
a major part of the cavity. The filling is cancellous, the cancelli
resembling those of P. australe and not those of PachyrukhosJ
The ventral wall of the sinus forms the ventral wall of the bulla.

The tympanic cavity is similar in shape to those of the inter-
atherids. As in Protypotherium, only the anterior vertical ridge
(remnant of the septum?) — the part forming the medial wall of the
Eustachian opening — remains. The tube (not detected when the
external structure was described) is entirely surrounded by the
tympanic, as in the forms previously described; its external orifice
is very deep dorso-ventrally. A styliform process similar to, but
larger, deeper and less distinctly differentiated than that of Pro-
typotherium, is present. The recessus epitympanicus and the pneu-
matic foramen are relatively smaller than in the interatherids.
Pseudotypotherium differs from Interatherium and Protypotherium in
having a shallower sulcus tympanicus, but agrees with them in
having a weak crista tympanica.

The position of the posterior carotid foramen in the posterior
wall of the bulla has already been described and figured (Patterson,
1934a). Neither the anterior carotid foramen nor the tympanic
aperture of the facial canal can be seen, probably as a result of slight
distortion. The openings of the various venous foramina have been
described previously. Their arrangement is similar in all essentials
to the pattern characteristic of the order. The canal of Huguier for
the chorda tympani and the internal orifice of the stylomastoid
foramen are visible and in their usual positions.

Suborder Toxodonta Owen

Toxodontidae Gervais

Nesodon Owen (fig. 49).

The following account is chiefly based on F.M. No. P13105, a
young individual referred to N. imbricatus Owen. The specimen

* In a previous paper on the external characters of this specimen (1934a, p. 84)
I stated that the hypotympanic sinus was cancellous. The section shows that this
is not strictly correct since only the expanded medial wall shows this condition.

210 Field Museum of Natural History — Geology, Vol. VI

was collected by Mr. Abbott in the Santa Cruz beds on the Atlantic
coast twenty miles south of Rio Coyle, Santa Cruz.

Scott (1912, p. 137) has given a good account of the external
appearance of the bulla in young and adult specimens. ^ In young
individuals it is bladder shaped, and the long axis is directed antero-
posteriorly; in adults it has a mammillate shape, and the long axis
is dorso- ventral. These changes in external appearance are accom-
panied by internal changes, as will appear below in the description
of an adult specimen of the closely related Adinotherium. In both
young and old individuals the antero-internal corner of the bulla
extends forward in a flat triangular plate which overlies part of the
alisphenoid, and there is a conspicuous styliform process extending
antero-internally from the lateral wall.

The bulla as a whole has a dorso-ventro-internal alignment; the
hypotympanic sinus being ventral and somewhat medial and anterior
to the tympanic cavity. The hypotympanic sinus is hollow and egg-
shaped with the narrow end pointing forward. The plate overlying
part of the alisphenoid, which was mentioned above, is formed by
an extension of its anterior wall. The walls of the sinus form the
greater part of the exposed portion of the bulla. The sinus is par-
tially separated from the tympanic cavity by an inconspicuous
septum. This structure is much smaller than the corresponding one
in the hegetotherids, being little more than a ridge. It is situated
ventral to the sulcus tympanicus and is thus almost horizontal — a
further difference from the hegetotherid septum.

The tympanic cavity is approximately equal in volume to the
hypotympanic sinus, but is much more irregular in shape. A recess
extends medially above the posterior extremity of the sinus behind
the opening into this cavity. A second, much smaller recess extends
anteriorly on the inner side of the medial wall of the Eustachian
tube. The anterior carotid foramen opens into this cavity, as in the
interatherids. The Eustachian tube is large, situated at the anterior
extremity of the tympanic cavity, and opens externally below the
posterior portion of the very large foramen ovale. It is surrounded
by the tympanic. The lateral wall of the tympanic cavity is very
thick and dense, and projects freely in an antero-internal direction
as the laterally compressed styliform process — a great contrast to
the decidedly sessile process of the interatherids and typotherids.

* It should be noted that Scott identified the bony plate which separates the
postglenoid and post-tympanic processes of the squamosal, and unites with the
lateral wall of the bulla, as a part of the mastoid process. Actually this plate is
the crista meati of the tympanic.

Ear Structure in Notoungulates

211

Below the Eustachian tube the styliform process and the lateral
wall of the hypotyropanic sinus do not fuse. The recessus epitym-
panicus is fairly deep, and the foramen pneumaticum is small in
comparison with the size of the hypotympanic sinus. The sulcus
tympanicus is poorly defined and very shallow; it is inclined ventro-

hypt. sin.

ffov.

sty.pr

^p.c.f?

-fen.coch.
-p.occ.pr
-v.pr. hy.

cr. m.
f — pa e.

a.c.f i\

ap t -tym. can. Fac '

tynn. cav.

pgl.e

' Fig. 49. Nesodon imbrieatu* Owen (juv.). F.M. No. P13105. Ventro-lateral view of auditory

region of right side. Dotted areas represent sectioned bone. For abbreviations see page 227. X 1/1.

internally and situated directly above the septum. A faint ridge
is the only vestige of the crista tympanica. The recessus meatus is
shallow. The promontorium is large, and the fenestra vestibuli and
fenestra cochlearis are similar to those of the forms previously
described.

The posterior carotid foramen is quite large and is situated in
the posterior wall of the bulla lateral to the foramen lacerum pos-
terius. As in all forms described here, hegetotherids excepted, both
foramina share a large common external orifice. The posterior carotid
entered the bulla lateral to the recess above the posterior
extremity of the hypotympanic sinus, its orifice being separated
from the recess by a vertical ridge of bone. No sulcus caroticus can

212 Field Museum^ of Natural History — Geology, Vol. VI

be detected. At the anterior extremity of the promontorium, and
medial and sHghtly posterior to the internal orifice of the Eustachian
tube, is a rather large foramen, deep dorso-ventrally, which leads
antero-internally to the cranial cavity to open there into a recess
near the exit of nerve V^ I regard this as the anterior carotid fora-
men. At its tympanic orifice it is bounded medially by the periotic
and laterally by the tympanic (and possibly by the alisphenoid also,
although this point can not be determined certainly from the speci-
men). This foramen is undoubtedly homologous with the one
similarly situated in Protypotherium. I have not seen any foramen
corresponding to the "lateral" foramen of Protypotherium described
above. There might, however, be such a structure within a small
recess above and behind the Eustachian tube.

The apertura tympanici canalis facialis opens at the posterior
extremity of the small recess just mentioned. As in Hegetotherium,
it is slightly dorsal and anterior to the fenestra vestibuli and is
bounded above by the tympanic, below by the periotic. A sulcus
facialis was present but the posterior part is not preserved in the
specimen. The internal orifices of the stylomastoid foramen and the
canal of Huguier appear to be in the same positions as in Hegetotherium.

Adinotherium Ameghino (fig. 50).

The description is taken from F.M. No. P13110, a young adult
referred to A. ovinum (Owen) and collected by Mr. Abbott in the
Santa Cruz beds on the banks of the Rio Coyle inlet, Santa Cruz.

As stated above, there are some interesting differences in pro-
portion from the young Nesodon. The hypotympanic sinus is oval
in shape with its long axis in the dorso-ventral direction, thus con-
forming to the external shape of the bulla. In correlation with the
increase in depth of the hypotympanic sinus in the adult, its wall
forms a greater part of the lateral wall of the bulla than it does in
young specimens. The tympanic cavity appears to be relatively
smaller in comparison with the size of the hypotympanic sinus than
in the young Nesodon, and is even more dorsal in position. In adult
toxodontids, therefore, there are three chambers one above the other
participating in the auditory structure — the hypotympanic sinus,
the tympanic cavity, and the epitympanic sinus. With regard to
the function of the sinuses I can do no more than follow the sug-
gestion of Gregory (1920, p. 164) that the hypotympanic sinus
probably functioned as an organ of resonance. Structurally, the
bulla of the adult Adinotherium differs so little from that of the young

pgl. £

cr. m.

can. /iug:^\--

^/y./7r— ____

sept/ 1 hP^^'"

'tym.cav.

.-^cf

crl cai^

I at cer^'^

ven. sin.

Fig. 50. Adinolherium ovinum (Owen). Dotted areas represent sectioned bone. For abbrevi-
ations see page 227. X 1/1. a, F.M. No. Pl.3110. Ventral view of auditory region of left side. b.F.M.
No. P13107. Ventro-medial view of section showing sinuses.

213

214 Field Museum of Natural History — Geology, Vol. VI

Nesodon that description is unnecessary. The styHform process is
generally more reduced in Adinotherium.

A very young specimen of the same species, F.M. No. P13107,
collected by Mr. Abbott five miles south of Coy Inlet shows some
interesting details. The bulla, periotic, basioccipital, occiput, and
part of the squamosal are missing. The resulting section shows
perfectly the large hollow epitympanic sinus and the relatively
small foramen pneumaticum connecting it with the recessus epitym-
panicus. Also, antero-medial to the epitympanic sinus appears
the large venous sinus into which open the various vascular foramina
bordering the squamoso-parietal suture, the postglenoid foramen,
the mastoid foramen, and the large cerebral venous sinuses, to be
described in another paper of this series, one of which is particularly
well shown on this specimen. The epitympanic sinus is surrounded
by this venous system but is not perforated by any member of it.
Simpson has described similar conditions in Oldfieldthomasia (1936,
pp. 27, 28).

Notohippidae Ameghino
Rhynchippus Ameghino.

Three specimens belonging to this genus are available, F.M.
Nos. P13410 and P13420 referred to R. equinus Ameghino, and A.
M. No. 29555 referred to R. pumilus Ameghino, but no one of them
is in a condition to permit detailed study. The first two were col-
lected by Mr. Abbott, the third by the first Scarritt Expedition; all
are from the Deseado beds at Cabeza Blanca, Chubut, Argentina.

The major structural features appear to be essentially similar to
those of Nesodon and Adinotherium. There is a large hollow hypo-
tympanic sinus which is ventral in position, and a smaller, more
dorsal tympanic cavity. The hypotympanic sinus conforms to the
external shape of the bulla and is approximately equal in length
and depth. The anterior extremity of the wall is thickened. The
internal faces of the walls are smooth in R. equinus, but in R. pumilus
they bear a number of thin bony ridges. The posterior carotid
foramen opens into the posterior wall of the bulla. No further
details of the carotid circulation can be detected in the specimens
available. In a previous description of the external structure of the
auditory region of R. equinus, F.M. No. P13410 (Patterson, 1932,
p. 13, fig. 3a), an opening was shown between the bulla and basioccip-
ital a little anterior to the foramen lacerum posterius on both sides
of the skull. A similar opening occurs on one side of F.M. No.

Ear Structure in Notoungulates 215

P13420. I believe these to be artifacts, but even if they are natural
it seems doubtful that they bear any relation to the entocarotid.

In a previous paper (1932) I described what appears to be a
mastoid process in this genus and Homalodotherium. I also de-
scribed (p. 19) an adventitious bone exposed on the occiputs of
certain other forms. Simpson (1936) has determined the presence
of an adventitious bone (possibly two of them) in Oldfieldthomasia
by means of serial sections, and has suggested (p. 8) that *the sup-
posed mastoid in Rhynchippus and Homalodotherium is in reality
the adventitious element. Work on the internal structure of these
forms has revealed that this suggestion is correct, the mastoid process
of the periotic being small and closely applied to, but distinct from
the adventitious bone.

Leontiniidae Ameghino

Ancylocoelus Ameghino (fig. 51).

I have already given some notes on a specimen of Ancylocoelus
frequens Ameghino^ (1932, pp. 15, 17, fig. 46), but these may be
added to in the light of the broader comparisons made in this paper.
The specimen, F.M. No. P14715, was collected by Mr. Sternberg
in the Deseado beds at La Flecha, Santa Cruz, Argentina. The
specimen consists of the squamosal and parts of the alisphenoid and
tympanic, and exposes the middle ear in dorso-ventral transverse
section. The dorso-ventral superposition of the epitympanic sinus,
tympanic cavity, and hypotympanic sinus is well shown, the arrange-
ment being very similar to that of the Toxodontidae. The very
small horizontal septum is shown in section directly beneath the
small, shallow, postero-medianly facing sulcus tympanicus, a further
resemblance to the toxodontids. There is no trace of a crista tym-
panica. The small pneumatic foramen to the epitympanic sinus is
visible above the sulcus tympanicus. The hypotympanic sinus, or
what remains of it, is hollow. The venous sinus is well shown on the
medial side of the epitympanic sinus. The figure shows one of the
vascular foramina along the squamoso-parietal suture opening into
it, and an opening, evidently for the passage of the lateral cerebral
venous sinus, leading medially from it. The course of the stylomas-
toid foramen is well shown. Its internal orifice is above the posterior

' Described as Colpodon sp. and the horizon given as Colhue-Huapl in the paper
cited. This error has been explained and corrected elsewhere (Patterson, 1935,
pp. 165-166).

I

216 Field Museum of Natural History — Geology, Vol. VI

<zpt sin ^^

3-ty.f:

/^■°''! hyphsin.

/ sept ^ C/r/77.

sulc.'tym. V pr. hy.

Fig. 51. Aneylocoelus jrequens kxae^vao. F.M. No. P14715. Posterior view of auditory region
of right side. Dotted areas represent sectioned bone. Occipital elements lacking. For abbreviations
see page 227. X 2/3.

"leg" of the sulcus tympanicus, apparently the usual position in the
order.

Leontinia Ameghino.

A description of the external structure of the auditory region of
this genus will be given in a paper now in preparation on the struc-
ture and affinities of the family. The internal structure, as far as
revealed by the available material, is almost identical to that of the
toxodontids.

Suborder Entelonychia Ameghino

Homalodotheriidae Ameghino
Hotnalodotherium Flower (figs. 52, 53).

The magnificent specimen of H. cunninghamt Flower,^ F.M. No.
P13092, collected by Mr. Riggs in the vicinity of Cape Fairweather,
Santa Cruz, and previously described in several papers, was further
prepared in the hope that it might reveal some details of the middle
ear structure. The bullae had been broken after burial and the first
preparator had merely added plaster to the broken edges. Mr.

1 This specimen was identified as segoviae by Professor Scott,
on page 225.

See addendum

Ear Structure in Notoungulates

217

Quinn, however, found upon removal of this plaster that the ventral
walls had been crushed up into the hypotympanic sinus and that
nearly all the pieces were preserved. These he skillfully fitted
together, thus giving an accurate idea of the external form. The
bulla is very nearly as deep as long, and is egg-shaped in ventral

Fig. 52. Homalodotheriiim eiinninghami Flower. F.M. No. P13092. Ventro-lateral view of
auditory region of left side. Dotted areas represent sectioned bone. Paroccipital process weathered
ofl. Bulla slightly displaced anteriorly. For abbreviations see page 227. X 3/4.

view, the narrow end pointing forward. Restoration of the bullae
has revealed some other features of interest in the surface
morphology.

Lateral to the posterior half of the styliform process is a blunt
process. The lateral portion of this is formed by the squamosal,
the medial and deeper portion by the alisphenoid. The latter bone
extends postero-externally in a fairly wide strip which forms the
greater part of the lateral border of the fissura Glaseri. Beyond the

218 Field Museum of Natural History — Geology, Vol. VI

blunt process it contracts to a thin strip of bone which reaches
nearly to the end of the fissure. The posterior extremity of the
fissura Glaseri is formed by an extension from the tympanic, an
exceptional feature. I have previously described (1932, p. 8) the
very complicated crista meati of this specimen. That portion of the
crest which partly covers the distal end of the postglenoid process
extends dorsally up the medial side of this process and then flares
out slightly just behind the blunt process described above (fig. 52).
Medially this extension inclines sharply upward to form a groove
behind the fissura Glaseri and lateral to the wall of the bulla with
which the extension is continuous. The squamosal is thus entirely
excluded from the fissura Glaseri. I have not encountered a com-
parable condition in any other notoungulate. In view of these
hitherto concealed structures a new figure of the ventral view of the
skull is given (fig. 53). This supersedes the figure previously pub-
fished by me (1934b, fig. 23).

The hypotympanic sinus is extremely large and forms the visible
part of the bulla. As in the Toxodonta, it is ventral to the tympanic
cavity, in fact rather more so than in that group since it has a greater
lateral extension. Its outstanding peculiarity lies in the rugose,
partly cancellous walls, and the low septa which radiate from the
opening communicating with the tympanic cavity. On the dorsal
parts of the posterior and lateral walls there are a number of small
septa, some of which begin at the opening while others arise further
down^the walls. None of these are very long. On the ventral parts
of the posterior and lateral walls the septa become fewer and lower,
and small inwardly projecting processes resembling miniature
stalagmites appear. On the anterior portion of the floor of the sinus
two major septa extend forward from the opening. One of these
proceeds anteriorly, the other antero-internally. Between them at
their extremities other smaller processes occur, the whole forming an
intricate network of septa and cellulae in the antero-internal part
of the sinus. The medial half of the ventral wall bears large irregular
septa. The fine septa on the walls of the sinus of Rhynchippus
pumilus constitute the only approach to this condition in any of the
other genera studied. In the vicinity of the opening to the tympanic
cavity, the roof of the sinus does not rest directly upon the ventral
surface of the promontorium, but is separated from it by a shallow
recess. As in the Toxodonta, the tympanic cavity and hypotym-
panic sinus are partially separated by a small horizontal septum
lying beneath the sulcus tympanicus.

pgi-^-.-:

Fig. 53. Homalodotherium cunninghami Flower. F.M. No. P13092. Cross-hatched areas
restored from Scott's figure. Left bulla slightly displaced anteriorly, pi., pterygoid; p., vomer; for
other abbreviations see page 227. X 1/3.

219

220 Field Museum of Natural History — Geology, Vol. VI

The tympanic cavity, although not fully revealed, appears to
be definitely smaller than the hypotympanic sinus. A well-developed
freely projecting styliform process is present which, as in the toxo-
donts, does not unite with the lateral wall of the hypotympanic
sinus. The promontorium appears to be relatively smaller than in
the other forms described.

The posterior carotid foramen and the external orifice of the
stylomastoid foramen have been described previously. The external
opening of the canal of Huguier is bounded posteriorly by the tym-
panic extension described above. No other details of the circulatory
or nervous systems are revealed, but they are doubtless similar to
those of the toxodonts.

AUDITORY OSSICLES

The most disappointing feature of the present study has been
the small number of ossicles encountered. Only five of these bones
were found: An incus from a specimen of Nesodon imbricatus Owen,
F.M. No. P12064, collected by Mr. H. T. Martin at Rio Gallegos,
Santa Cruz; and the malleus and incus from two individuals of
Adinotherium ovinum (Owen), F.M. Nos. P13110 (described above)
and P13231, collected by Mr. Abbott on the Atlantic coast twelve
miles south of Rio Coyle, Santa Cruz.

Malleus (fig. 54). — The head as a whole is very robust, and
extends forward far in advance of the dorso-ventral axis of the neck.
The portion of the head above the articular surface is large, smooth,
convex, and irregularly oval in outline. It projects farther on the
internal than on the external side. The antero-posterior diameter is
rather greater than the transverse; the transverse diameter is greatest
posteriorly. The articular surface for the incus is approximately
one and one-half times as broad as it is deep. The two facets form
a deep concavity with an angle of about 120°, and are separated at
their centers by a groove. The upper has an irregular proximal
border and is excavated distally. The external half of the facet is
gently convex, the internal half gently concave. The lower facet
is not as wide transversely as the upper but is somewhat deeper; its
proximal border is arched to conform with the distal outline of the
upper facet. The entire surface is gently convex.

The neck is straight, stout and short; the antero-posterior diam-
eter is slightly greater than the transverse. The external face bears
a small forwardly directed process situated midway between the
head and the processus brevis. A very small lamina is present on

Ear Structure in Notoungulates

221

the anterior face which does not project out from beneath the head.
The major portion of the lamina is situated beneath the anteriorly
projecting portion of the head and is strongly convex ventrally.
This portion is continuous with a smaller and straighter part which
extends anteriorly from the neck. There is no processus gracilis.
Between the head and the lamina is a deep concavity which con-
tinues distally as a groove in the internal face of the neck, ending in a

brev.

pr long:-

prbrey.

Fig. 54. Auditory ossicles of left side. For abbreviations see page 227. X 4/1. a, Adinotheriam
orintim (Owen). F.M. No. P13231. Medial view of malleus and incus, b, The same. Lateral view
of malleus and incus, e, Nesodon imbritatiig Owen. F.M. No. P12064. Anterior view of incus, d,
AdiTtotherium ovinum (Owen). F.M. No. P13231. Posterior view of malleus, e, The same. Anterior
view of incus.

foramen slightly below the level of the processus brevis. This process
is compressed antero-posteriorly and is quite prominent. No proces-
sus muscularis is visible on the part preserved.

The manubrium is almost entirely missing. The preserved
proximal extremity is considerably compressed ; its maximum trans-
verse diameter is directed postero-internally.

Incus (fig. 54). — This bone is so similar in Adinotherium and
Nesodon that separate descriptions are not necessary. The pro-
portionate size of incus to malleus is approximately the same as in
the primates. The body is stout and approximately one and one-
third times as deep as it is wide. The articular surface is an exact
counterpart, in reverse, of that on the malleus. A slight difference
occurs between Nesodon and Adinotherium in the contour of the

222 Field Museum of Natural History — Geology, Vol. VI

proximal border of the upper facet. A groove surrounds its internal,
proximal, and lateral margins. The external surface of the body is
nearly plane, the internal surface is rather more rounded. The
processus brevis is short, straight, and quite slender. In Nesodon it
is nearly circular in section; in Adinotherium slightly compressed
laterally. The processus longus is compressed antero-posteriorly,
concave anteriorly, and convex posteriorly. It is slightly bowed
anteriorly and the distal portion turns inward. In Adinotherium the
external and internal sides taper evenly towards the distal end, but
in Nesodon the internal side is very heavy and decidedly convex,
imparting to the process a considerable transverse diameter. The
distal extremity of the processus longus is unfortunately broken off
in all specimens. The two processes are not strongly divergent.

A careful perusal of the classic work of Doran (1878) has failed to
reveal any existing form that is closely comparable to Nesodon and
Adinotherium. In view of the taxonomic position of the Notoungu-
lata, this is not surprising. Before useful comparisons can be made,
knowledge of the ossicles of the Condylarthra and other primitive
ungulate orders will be necessary, a seemingly remote possibility at
present. It may perhaps be emphasized that the ossicles described
above show no important resemblances to, and many differences
from those of the Hyracoidea {Procavia has been directly compared).
This is of some interest in connection with the now generally dis-
credited hypothesis of direct typothere-hyracoid relationships.

THE SEPTUM IN THE BULLA

Among the various septa which may occur in the bulla is one
known as the septum bullae formed by union of ecto- and ento-
tympanic elements. This septum "always shows a free upper mar-
gin, the rostral, under, and caudal margins being attached to the
bulla," and never "runs inward from the sulcus tympanicus" (Van
der Klaauw, 1931, p. 210). The septa of Hegetotherium and Pachy-
rukhos fulfill these requirements so completely that I had no hesita-
tion in stating in a previous paper (1934a, p. 84) that the bulla was
compound. Conditions in other genera, however, are not identical.
The septum is not present in the Interatheriidae and Tjrpotheriidae,
although vertical ridges which might be remnants of it do occur.
In the Toxodonta and Homalodotherium the septum is greatly
reduced in size and horizontal in position. Provided that this small
ridge is not a neomorph, it is homologous with the hegetotherid
septum since in that family the hypotympanic sinus is lateral to the

Ear Structure in Notoungulates 223

tympanic cavity and the septum consequently vertical, while in
those forms in which the septum is horizontal the hypotympanic
sinus is below the tympanic cavity. The large size of the hypotym-
panic sinus in the Toxodonta and Ho-malodotherium, and its sharp
demarcation from the tympanic cavity, could be regarded as evidence
that it was formed by a separate element.

Oldfieldthomasia, the only Casamayor genus and the only notio-
progonian in which the mJddle ear structure is known, shows no
structure that could be definitely regarded as a septum bullae
(Simpson, 1936, p. 14). Since the Notioprogonia as a group are
ancestral to the surviving later families, it might be concluded that
the lack of a septum bullae in Oldfieldthomasia indicated that the
notoungulate bulla was not compound and that the septa described
in this paper were not due to union of ecto- and entotympanics.
On the other hand it is possible that Oldfieldthomasia has secondarily
lost the septum. Until the middle ear structure of other notioprogon-
ians is known, it will be impossible to arrive at a decision between
these alternatives. The hegetotherid septa and the very distinct
hypotympanic sinus of the Toxodonta nevertheless incline me to
believe that the bulla is compound. The evidence at present avail-
able, however, is not complete enough to justify my somewhat too
positive earlier statements.

SUMMARY

The external characters of the auditory region are comparatively
uniform throughout the order. It was therefore somewhat surpris-
ing to find, as this investigation progressed, that the internal struc-
ture showed greater diversification than had been expected. The
differences and resemblances between the various families are of
considerable taxonomic interest.

Characters common to all forms described. — Well-developed hypo-
tympanic sinus; Eustachian tube opening postero-lateral, or posterior
to the foramen ovale and situated at the anterior extremity of the
tympanic cavity (surrounded by the tympanic in all forms described
here in which this detail could be observed, but bounded dorsally by
the periotic in Oldfieldthomasia (Simpson, 1936, p. 17) ; epitympanic
sinus in the squamosal connected by the foramen pneumaticum with
the recessus epitympanicus; course of the facial nerve and chorda
tympani essentially as described by Simpson for Oldfieldthomasia and
above for Hegetotherium, probably only minor variations of this
arrangement occurring in the order; conspicuous postero-lateral
venous sinus situated antero-internal to the epitympanic sinus and

224 Field Museum of Natural History — Geology, Vol. VI

receiving cerebral venous sinuses, the postglenoid foramen or fora-
mina, the various venous foramina distributed along the squamoso-
parietal suture, and the mastoid foramen; small mastoid process
of periotic.

Some additions and subtractions may be necessitated by future
work, but it seems likely that these features will in the main be found
characteristic of the order as a whole.

Distinctive characters of the Hegetotheriidae. — ^A vertical septum
in the bulla between the tympanic cavity and the hypotympanic
sinus, the septum being attached to the bulla anteriorly, ventrally
and posteriorly; no styliform process; relatively well-developed
crista tympanica; internal carotid artery did not traverse the bulla.

Distinctive characters of the Interatheriidae and Typotheriidae. —
No septum in the bulla (vertical ridges that may be vestiges of this
structure have been noted above), the separation of the tympanic
cavity and the hypotympanic sinus consequently not as distinct as in
other groups; a tendency in both families towards a filling in of the
hypotympanic sinus by cancellous enlargement of its medial wall;^
styliform process present or secondarily lost (e.g. Interatherium
robustum), when present appressed to the anterior end of the bulla;
low crista tympanica; internal carotid artery traversed the bulla.

In a previous paper (1934c, pp. 135-136) I suggested on the basis
of certain external characters that the Interatheriidae and Typo-
theriidae were perhaps more closely related to each other than to the
Hegetotheriidae. It was very gratifying therefore to find that this
suggestion was strongly supported by the middle ear structure.
Derivation of the Typotheriopsis or Pseudotypotherium middle ear
from a type such as that of Protypotherium australe would involve
only a slight dorsal migration of the tympanic cavity.

Distinctive characters of the Toxodonta and Homalodotherium. —
Hypotympanic sinus as large as or larger than the tympanic cavity
and ventral to it, these cavities forming with the epitympanic sinus a
dorso-ventral row of three interconnecting chambers; a small hori-
zontal septum between the tympanic cavity and the hypotympanic
sinus; a projecting, laterally compressed styliform process usually
present; internal carotid artery traversed the bulla.

The present study was completed at this time in an attempt to
discover deep-seated differences between the Toxodonta and En-

1 The medial wall is also enlarged in Pachyrukhos moyani, but, as stated above,
the cancelli are of a different type.

Ear Structure in Notoungulates 225

telonychia which might aid in determining the subordinal position
of the leontiniids. The notohippids were found to resemble the
toxodontids in middle ear characters, while the leontiniids were
found to agree more closely with the toxodontids in both external
and internal characters than with any other family. It appeared
likely that as far as the ear was concerned the Leontiniidae would
fall naturally into the suborder Toxodonta. Homalodotherium was
then sectioned in the confident expectation of finding characters at
least as striking as those separating the "toxodont" middle ear type
from either of the typotherian types. Greatly to my surprise, how-
ever, this genus, so unique in skeletal characters and so different
from the toxodontids in the external appearance of the auditory
region, did not differ from the Toxodontidae in any internal charac-
ter of importance. Homalodotherium is if anything somewhat
further advanced along the "toxodont" line of specialization than
any other form studied, in that it has a larger, more laterally extended
hypotympanic sinus. The rugose interior of the sinus is not de-
veloped in any of the toxodontids or leontiniids studied, but is present
to a lesser degree in the notohippid Rhynchippits pumilus. It may
be stated, therefore, that on the basis of middle ear structure, a highly
important criterion,* the homalodotherids stand in about the same
relation to the Toxodonta as the chalicotherids do to the less aberrant
perissodactyls. The effect that this finding may have on the current
classification of the Notoungulata or on the subordinal status of the
Leontiniidae can not be decided at present.

ADDENDUM

Professor Scott in his memoir on the Field Museum skeleton of
Homalodotherium (Field Mus. Nat. Hist., Geol. Mem., I, p. 7, 1930)
referred the specimen to H. segoviae (Ameghino), a reference
which has been repeated by me in preceding papers of this volume.
It was decided recently to mount the specimen, and during the
reconstruction of the eroded cranial roof some questions of propor-
tion led to an examination of the specific determination. As a result,
it was found that the skeleton should be referred to H. cunninghami,
the genotypic species, rather than to segoviae. The most striking
distinction between the two species lies in the greatly reduced P-
of segoviae (Scott, 1912, p. 284, pi. 28). In P13092 this tooth is
well developed, as in cunninghami (fig. 51; Scott, 1912, pi. 29, fig. 3).
The difference in the condyles mentioned previously by me (1934b,
p. 117) may well prove to be a further specific distinction.

REFERENCES

DORAN, H. G.

1878. The Mammalian Ossicula auditus. Trans. Linn. Soc. Zool., (2), 1,
pp. 371-497, pis. 58-64.

Gregory, W. K.

1920. On the Structure and Relationships of Notharctiis, an American Eocene
Primate. Mem. Amer. Mus. Nat. Hist., n.s., 3, pp. 45-243, pis. 23-59,
84 text figs.

KlAAUW, C. J. VAN DER

1931. The Auditory Bulla in Some Fossil Mammals with a General Introduction
to This Region of the Skull. Bull. Amer. Mus. Nat. Hist., 62, pp. 1-352, 18
text figs.

Patterson, B.

1932. The. Auditory Region of the Toxodontia. Field Mus. Nat. Hist.
Geol. Ser., 6, pp. 1-27, 5 text figs.

1934a. The Auditory Region of an Upper Pliocene Typotherid. Ibid., pp.

83-89, 3 text figs.
1934b. Cranial Characters of Homalodotherium. Ibid., pp. 113-117, 1 text fig.
1934c. Trachytherus, a Typotherid from the Deseado Beds of Patagonia.

Ibid., pp. 119-139, 5 text figs.

1935. A New Argyrohippus from the Deseado Beds of Patagonia. Ibid., pp.
161-166, 2 text figs.

Roth, S.
1903, Los ungulados sudamericanos. An. Mus. La Plata, 5, pp. 1-36, pis.
1-4, 3 text figs.

Scott, W. B.

1912. Toxodonta of the Santa Cruz Beds. Entelonychia of the Santa Cruz
Beds. Repts. Princeton Univ. Expeds. Patagonia, 6, pp. 111-300, pis. 12-30,
30 text figs.

Simpson, G. G.

1933. Braincasts of Two Typotheres and a Litoptern. Amer. Mus. Nov.,
No. 629, pp. 1-18, 3 text figs.

1936. Structure of a Primitive Notoungulate Cranium. Ibid., No. 824, pp.
1-31, 11 text figs.

Sinclair, W. J.

1909. Typotheria of the Santa Cruz Beds. Repts. Princeton Univ. Expeds.
Patagonia, 6, pp. 1-110, pis. 1-11, 16 text figs.

226

ABBREVIATIONS ON FIGURES

a.c.f.

al.

a.m.

apt.tym.can.fac.

(?)br.sty.art.

c.f.

can. Hug.

car.f.

cr.m.

cr.tym.

crl.cav.

E.f.

ept.sin.

f.l.p.

f.ov.

f.pn.

fen.coch.

fen.vest.

h.

hypt.sin.

lam.

lat.cer.ven.sin.

p.a.e.

p. c.f.

par.-occ.sut.

per.

pgl.f.

pgl.pr.

p.-l.ven.sin.

p.occ.pr.

pr.

pr.brev.

pr.long.

prom.

rec.ept.

sept.

sq.

sty.f.

sty.pr.

sulc.fac.

sulc.tym.

tym.

tym.cav.

v.pr.hy.

v.r.

vascf.

ven.f.

xp.

anterior carotid foramen

alisphenoid

auditory meatus

apertura tympanici canalis facialis

(?)groove for branch of stylomastoid artery

condylar foramen

canal of Hug^uier

carotid foramen

crista meati

crista tympanica

cranial cavity

Eustachian tube

epitympanic sinus

foramen lacerum posterius

foramen ovale

foramen pneumaticum

fenestra cochlearis

fenestra vestibuli

head

hypotympanic sinus

lamina

lateral cerebral venous sinus

mastoid process

neck

porus acusticus externus

posterior carotid foramen

parieto-supraoccipital suture

periotic

pMastglenoid foramen

postglenoid process

postero-lateral venous sinus

paroccipital process (sectioned)

process on neck of malleus

processus brevis

processus longus

promontorium

recessus epitympanicus

septum in the bulla

squamosal

stylomastoid foramen

styliform process

sulcus facialis

sulcus tympanicus

tympanic

tympanic cavity

vagina processus hyoidei

vertical ridges in the bulla, possibly vestiges of the septum

unidentified foramina, probably vascular

venous foramina opening externally on or near the squamoso-

parietal suture
posterior adventitious bone

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