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The Sesamoid Articular

A Bone in the Mandible of Fishes

BY

EDWIN CHAPIN STARKS

Assistant Professor of Zoology

WITH FIFTEEN TEXT FIGURES

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[Issued March 31, 1916]

STANFORD UNIVERSITY, CALIFORNIA

PUBLISHED BY THE UNIVERSITY

IQl6

STANFORD UNIVERSITY
PRESS

THE SESAMOID ARTICULAR: A BONE IN THE
MANDIBLE OF FISHES

CONTENTS

INTRODUCTION

SYNONYMY

THE ADDUCTOR MANDIBULAE MUSCLES

THE SESAMOID ARTICULAR

SUMMARY

INTRODUCTION

The sesamoid articular is a small bone on the inner surface of the
articular of fishes giving attachment to a stout tendon from the adductor
mandibulae muscles.

This investigation was undertaken to ascertain in what groups of
fishes it might occur; what its variation and condition might be in dif-
ferent groups ; and to give the early history of the literature, which recent
authors seem to be ignorant of.

Originally it was intended to work on the homologies of the adductor
mandibulae muscles in order better to establish the identity of the sesa-
moid articular and its tendon ; but after considerable work (done mostly
at the Zoological Station at Naples in January and February of 1915), it
appeared that the muscles are so variable in size, position, and relationship
to each other that the sesamoid articular and its tendon were more valu-
able in identifying the muscle than the reverse.

A few of the drawings and brief descriptions of the muscles are
here presented, and show their diversity of form and position. Many
other fishes were worked on, but to report on all of them does not appear
to be of advantage to the present inquiry.

The first notice of the sesamoid articular occurs in Cuvier and
Valencienne's Histoire Naturelle des Poissons. Cuvier found it in the
perch, and published such a picture of it that there can be no question as

4 THE SESAMOID ARTICULAR

to its identity. He called it the Operculaire (number 37 of his picture),
homologizing it with a bone in the jaw of reptiles that he called by the
same name, supposing the reptile bone homologous with the true opercle
bone of fishes, though in the fish he also uses this name (number 28) for
the bone commonly so called.

In 1846 Owen, in his Lectures on Comparative Anatomy and Physi-
ology of the Vertebrates, published a picture of the mandible of the Sudis,
Arapiama gigas, with the following remarks: "The great Sudis and
the Polypterus have the splint-like plate along the inner surface of the
ramus, answering to that which Camper and Cuvier have unfortunately
called 'operculaire' in the mandible of reptiles, but to which I have given
the name of 'splenial' to prevent the confusion from the synonymy with
the true opercular bone of fishes." Later in his Comparative Anatomy
Owen included Amiatus with the other two forms, and adds : the splenial
"supports teeth and develops a coronoid process."

It thus appears that Owen's splenial is the bone that usually bears
that name in the Ganoids. It need scarcely be added that no homology
exists between the splenial, which is clearly a dental cement bone, and
the sesamoid articular (Cuvier's Operculaire).

Giinther, in his Introduction to the Study of Fishes (p. 91), makes
a similar mistake when he refers to the os operculare as a synonym of the
splenial as follows. "The splenial or os operculare, which is situated on
the inside of the articulary."

Ridewood (Linn. Soc. Jour., XXIX, p. 267), in writing of the
mandible of the Sudis, says: "Although the bony lamina that bears the
teeth occupies the position of the splenial bone, it is not a distinct plate
of bone as might be concluded from the remark of Owen." There can
be little question but that Owen referred to this inseparable toothed plate,
rather than to a sesamoid articular. Dr. Ridewood reports the sesamoid
articular not present.

Owen, in a table of synonyms that he published in his Lectures,
includes the name subvomeral of Geoffrey St. Hilaire. I fail to find the
term in the paper Owen cites (Ann. des. Sci. Nat., Ill, 1824), and there-
fore can not be sure whether it is the homolog of the sesamoid articular
or of the splenial; but if of the former it is an earlier account of the
bone than the one given by Cuvier.

Agassiz also uses the term Operculaire for the splenial of Polypterus
and Lepisosteus (Poissons Fossiles, vol. 2, part 2, pp. 20 and 42, pi. B
and C, 1843), but in the Sudis (Piscium Brasiliensium, pi. B, 1829) he
applies operculaire to an entirely different bone, that Owen in the same

INTRODUCTION 5

form called the surangular — confusing it with the surangular of Ganoids
— and that Ridewood says, is "merely the endosteal articular displaced."

Erdl, in his work on Gymnarchus, labeled the sesamoid articular, in
both German and Latin, the kronenforsatz des Unterkiefers, pars coro-
noidea mandibulae. He thus, doubtless, as Cope did later, considered
it the homolog of the coronoid of reptiles.

Bridge in 1877 described the bone in Amiatus that I have herein
homologized as the sesamoid articular. He termed it simply ossicle C,
but since that time every anatomist who has worked on Amiatus has
referred to it as "ossicle C of Bridge."

The last author to write of this bone was Ridewood (Proc. Zool.
Soc. Lond., 1904, p. 72), whom I follow in the use of the term 'sesamoid
articular' as the best suggested for it. It is the Anglicized form of
Vetter's Sesamoidverknocherung. He gives a history of the literature
of the bone since the time of Cope, which I need not here repeat. Among
the papers touched upon he gives undeserved prominence to a hastily
prepared foot-note published by me in 1899 (Proc. U. S. Nat. Mus.,
XXII), in which I listed various unrelated forms that possessed a sesa-
moid articular. My material was such prepared skeletons as happened to
be at hand, and the note was published purely with the object of showing
the worthlessness of the bone in taxonomy as used by Cope. As my
discussion was based on Cope's paper I followed him in the use of
the term 'coronoid bone' without thought of homology.

Most of the examples I reported upon were adult; and because the
bone happened to be missing in a young individual of Mugil, the question
was brought up as to whether it was not the ossified end of the tendon,
that is always attached to it, developed with age. This seemed more
probable at that time, as I was investigating the skeleton of Dallia in
which the posttemporal ligament ossifies rather late in life. In the young
Mugil referred to the bone was doubtless lost in preparing the skeleton,
for I find it well developed in much smaller individuals that those re-
ported upon.

It has seemed most convenient for the needs of this paper to follow
no scheme of grouping. The captions are not at all coordinate in value,
nor are they consistent in terminology. For instance, the small orders
of Ganoids together with the lung fishes are under one caption. "The
eels" includes two orders; "the clupeoid fishes" is of superfamily rank;
and the spiny-rayed fishes are grouped under such comprehensive cap-
tions as "the blennies" or "the mailed-cheeked fishes," or even under
family names.

6 THE SESAMOID ARTICULAR

Where the term 'endosteal' or 'ectosteal part of the articular' is used
it does not mean that these elements are necessarily separate. Nearly
always the anterior end of the endosteal part remains distinct from the
ectosteal part, though no trace of suture may remain. The endosteal
part forms the surface for the articulation of the quadrate, and usually
projects forward in a process, which I have herein referred to as the
endosteal process. The process abruptly becomes Meckel's cartilage
(or more correctly Meckel's cartilage abruptly ossifies to form the endo-
steal process). Often the anterior part of the endosteal process is of
the same size and shape as Meckel's cartilage, and the line of demarca-
tion between the two is difficult to appreciate, by sight alone, until the
latter begins to dry. The ectosteal part of the articular, usually herein
called the ectosteal plate, lies outward from the endosteal part and forms
the greater part of the bone. It is the continuation of the part articu-
lating with the dentary, sheltering Meckel's cartilage on the outer surface.

I may here point out the danger of reporting on the absence of the
sesamoid articular in material that the investigator has not prepared for
himself, for it is often so loosely attached to the other elements of the
mandible, and so firmly to the tendon, that in cleaning the bones it is
easily removed with the latter. For instance, I found it in all of the
Clupeoid forms that I examined, though in several of them it has been
reported absent.

It was the original intention to investigate the development of the
bone; but from lack of good material it appears better to publish the
paper in its present form, reserving the development for the indefinite
future when, it is hoped, material shall have accumulated to illustrate
the various phases of it.

SYNONYMY

Operculaire Cuvier (C. & V. Hist. Nat. Pois., 1827).

Kronenforsatz des Unterkiefers, pars coronoidea mandibulae Erdl

(Abhand. Bayer Akad. Wiss., V— I, 1847).
Ossicle C. Bridge (Jour. Anat. Phys., XI, part 4, 1877).
Sesamoidverknocherung Vetter (Jena Zeitschr., XII, 1878).
Coronoid Cope (Proc. Am. Phil. Soc., XVII, 1878).
Addentary Gill (Proc. U. S. Nat. Mus., XVIII, 1895).
Sesamoid articular Ridewood (Proc. Zool. Soc. Lond., 1909).

THE ADDUCTOR MANDIBULAE MUSCLES

Scorpaena scrofa.

This short description and drawing is presented only to include a
common type. Dr. Allis, in his Cranial Anatomy of the Mail-Cheeked
Fishes (Zoologica Stuttgart, H, 57, 1909), has given a very complete,
and much better description.

A 2+3

Fig. i. — SCORPAENA SCROFA
A 1-2-3, muscles of Vetter ; Lap, levator arcus palatini ; max, maxillary ; T, tendon.

The superior part of the adductor mandibulae consists of two parts :
( i ) A dorsal muscle originating on the preopercle and running forward
to a broad tendinous band along its entire front, which at its upper end
forms a tendon to the maxillary, and at its lower joins the tendons of
the ventral muscle to the mandible. (2) A ventral muscle originating
on the preopercle (and on other bones anterior to it), and covered on its
upper part by the dorsal muscle. Anteriorly it is inserted on three tendons
running to the mandible ; the middle one of which is the sesamoid articu-
lar tendon. This muscle represents A-2+ 3 of Vetter.* According to

*Vetter. Janaische Zeitschr., XII, 1878.

8 THE SESAMOID ARTICULAR

Dr. Allis that part of it that posteriorly runs internal to the levator arcus
palatini may represent A-3 and the part external to it A-2.

Dentex vulgaris.

The superior part of the adductor mandibulae muscle consists of
two parts : ( i ) A dorsal muscle which at first sight appears to be made
up of two muscles — an outer and an inner one. The outer part is strongly
attached to the maxillary at one end by a short stout tendon, and to the
preopercular ridge at the other end by a longer, but scarcely less stout,
tendon. Above the preopercular tendon projects the inner part. It
originates on the preopercle and runs downward and forward obliquely
behind the outer part, and is attached anteriorly to the lower edge of the
outer part, and to a thick sheet of tendon that covers the greater part

L<vp.

At

Fig. 2. — DENTEX VULGARIS
A 1-2-3, muscles of Vetter; Lap, levator arcus palatini.

of the inner surface of all of the muscles. The lower edges of the inner
and the outer parts are continuous, so that when the two are spread
out it is apparent that they are a single muscle folded upon itself. It
represents A-i of Vetter.

(2) A ventral muscle, which is larger than the dorsal muscle, origi-
nates on the preopercular ridge and the surface of the anterior adjoining
bones. The thick sheet of tendon, that covers much of the inner part of
this and the dorsal muscle, shallowly invades this muscle at about its
middle, and along the plane of this invasion the muscle may be rather

THE SESAMOID ARTICULAR 9

easily separated. The inner part thus separated may represent A~3 of
Vetter, and the outer part A-2.

The upper part of the sheet of tendon is free from the muscle and
at its anterior end helps to form the maxillary tendon. Continuous with
this sheet of tendon is the tendinous covering of the inferior part of the
adductor mandibulae, or the part confined to the mandible (A-co of
Vetter), which posteriorly sends back a bundle of fibers to the middle
of the quadrate. Dissecting this tendinous covering away a stout sesa-
moid articular tendon is found, which comes from the lower part of the
sheet of tendon, or the part covering the ventral muscle (A-2+3).
Attached to the articular below the sesamoid articular and below Meckel's
cartilage is a less compact bundle of tendinous fibers from the same source.
Crossing these two, and attached still lower on the articular, is another
bundle of fibers coming more directly from the upper part of the sheet
of tendon, or the part covering the dorsal muscle (A-i).

The levator arcus palatini muscle is very small, and no part of the
adductor mandibulae runs behind it.

, Tylosurus acus.

The superior portion of the adductor mandibulae is in two parts:
(i) An external part that is ventral to the internal muscle, instead of
dorsal to it as in Scorpaena. It originates on the preopercle and covers
a considerable portion of the levator arcus palatini and of the internal

L <vp.

so.

Al

Fig- 3-— TYLOSURUS ACUS
A 1-2-3, muscles of Vetter; Lap, levator arcus palatini; sa, sesamoid articular.

muscle. Its tendinous surface fibers on its lower part converge and are
joined directly to the sesamoid articular, while those of the upper part
are attached to the tendon which descends from the dorsal internal
muscle and borders its anterior edge. (2) The portion referred to
above as the dorsal internal muscle has its origin on the anterior edge of.
the hyomandibular and the surface of the metapterygoid. Posteriorly

10

THE SESAMOID ARTICULAR

it is covered by the levator arcus palatini. It sends down a stout tendon
bordering the posterior orbital margin, and the anterior edge of the
ventral muscle as described, and joins the sesamoid articular. As neither
of these muscles sends a tendon to the maxillary, and as each contributes
to the sesamoid articular, the homologies of the two parts are not very
clear. From their relation with the levator arcus palatini the outer ven-
tral muscle would seem to represent A-i of Vetter, and the dorsal inner
muscle A-2 (+3?).

Mugil cephalus.

The superior part of the adductor mandibulae muscle consists of
three parts: (i) A lower outer muscle which originates on the pre-
opercular ridge and is connected by a stout tendon to the maxillary (A-i
of Vetter). (2) A muscle situated above and partly under the first origi-
nates higher on the preopercular ridge, and lies outside of the levator
arcus palatini muscle (A-2 of Vetter). (3) The third is almost wholly

Fig. 4. — MUGIL CEPHALUS
A t-z-S, muscles of Vetter; Lap, levator arcus palatini; max, maxillary.

covered by the second, its posterior end lying under the levator arcus
palatini, and originating on the surface of the metapterygoid. The
second and third unite anteriorly in a stout sesamoid articular tendon.
The inferior, or mandibular part of the muscle (A-co of Vetter), which
is almost confined to the mandible, consists of two parts. The upper
part gives off a broad band of tendon which connects it with the front of
the quadrate, and it also sends a few fibers to join the sesamoid articular

THE SESAMOID ARTICULAR II

tendon. The lower part is longer and inward from the first. It runs
back to just in front of where the interhyal joins the face bones.

Leiognathus fasciatus.

This form is unique in having two equally stout tendons to the sesa-
moid articular. A large muscle (A-i of Vetter) springs from the pre-
opercle and covers the cheek. Its fibers converge to a point anteriorly
from which a stout tendon runs upward and forward to the maxillary,
and another downward and backward to the posterior end of the sesa-
moid articular. A long muscle (A-3 of Vetter) springing from under
the levator arcus palatini gives rise to a second tendon, which, crossing
the first tendon, reaches the sesamoid articular near its middle.

THE SESAMOID ARTICULAR

THE GANOIDS AND LUNG-FISHES.
Acipenser transmontanus, A. mikadoi.

In this genus I find only two bones : the very large dentary, forming
the greater part of the mandible, and a small splint of bone lying on the
inner surface of Meckel's cartilage. (In large specimens a mento-
meckelian bone is developed in the anterior part of Meckel's cartilage
and forms a third element.) From its position the small inner splint
might either represent the sesamoid articular or the splenial. The tendon,
however, of the adductor mandibulae (or at least the muscle that func-
tions as such) is attached to the upper edge of the cartilage between this
inner bone and the dentary, and though some outer fibers of the tendon
are attached to the edges of both bones, the main attachment would sug-
gest that no sesamoid articular were present and that the inner bone
represents the splenial. It is the bone called the coronoid by W. K. Parker
in his Structure and Development of the Skull in Sturgeons (Phil. Trans.,
1882, p. 173, pi. 18). In my specimens it is much larger than is shown
in Dr. Parker's plate, and is everywhere isolated from the dentary by
cartilage.

Lepisosteus platostomus, L. osseus.

Among the eight elements that make up the mandible of Lepisosteus
(two of which are presplenials) I find none that I can homologize with
the sesamoid articular, unless it be an inner shelf of the splenial in the
coronoid region. This is not in the typical region for the sesamoid
articular, being remote from Meckel's cartilage, and the only ground for
considering the possibility of this homology is that a stout adductor ten-
don is attached to it. A tendinous band also gives muscle attachment to
the edge of the supra-angular. I have examined several specimens of
various sizes down to 3^2 inches in length, but find no indication of this
inner splenial process as a separate element.

THE SESAMOID ARTICULAR 13

Amiatus calva.

Just above the posterior part of Meckel's cartilage are two small
bones in the typical position of the sesamoid articular. They were first
noticed by Bridge (Jour. Anat. Phys., vol. XI, 1877), who designated
them on his drawing of the mandible of Amiatus as "c" and "b" and so
referred to them without giving them names. To ossicle c is attached
a tendon from muscles that are probably homologous with parts of the
adductor mandibulae muscles in more highly specialized teleosts.* Dr.
Ridewood (Proc. Zool. Soc. Lond., 1904, p. 56) has failed to find these
two ossicles as separate bones and has homologized them with the endo-
articular. In two adult specimens I have at hand, however, these two
bones are very evident. Ossicle c is the sesamoid articular, and ossicle
b only represents the endoarticular.

Polypterus bichir.

In Polypterus I find no separate sesamoid articular, but on the inner
surface of the articular, just above Meckel's cartilage, in the typical place
for the sesamoid articular is a projecting shelf of bone to which a stout
adductor tendon is strongly attached. My only specimen is one 18 inches
in length. In the young I should confidently expect to find this process
as a separate sesamoid articular.

Ceratodus forsteri.

Of the lung-fishes I have only Ceratodus in which I find no sesamoid
articular. A big bundle of tendons gives muscle attachment to both the
ectoarticular and splenial just behind the mandibular tooth in the coro-
noid region. In the other lung-fishes there is no bone in published descrip-
tions or figures that could be homologized with the sesamoid articular.

THE CLUPEOID FISHES.

Dr. Ridewood, "On the Cranial Osteology of the Fishes of the Fami-
lies Elopidae and Albulidae" (Proc. Zool. Soc. Lond., 1904), reports the
sesamoid articular present in Elops saurus and in Albula vulpes, not
present in Pterothrissus gissu (Bathythrissa dorsalis), and in Me galops
cyprinoides he does not mention it, but he states that except for propor-
tions it resembles Elops.

*For a discussion of these see Allis, Cranial Muscles and Nerves of Amia
calva. (Jour. Morph., XII, 1897, PP- 554, 5&) and 752.)

14 THE SESAMOID ARTICULAR

In his paper "On the Cranial Osteology of the Fishes of the Families
Mormyridae, Notopteridae, and Hyodontidae" (Jour. Linn. Soc., vol.
XXIX), he reports a sesamoid articular present in Notopterus kapirat
and Hiodon alosoides.

In his paper "On the Cranial Osteology of the Clupeoid Fishes"
(Proc. Zool. Soc. Lond., 1904) he finds a sesamoid present in Chirocen-
trus dorab, Alosa finta, and Chanos chanos, but fails to find any in Clupea
harengus, Dussumieria acuta, Engraulus encrasicholus, or Coilia nasus.
In Dorosoma erebi he mentions neither its presence nor absence.

In his paper "On the Cranial Osteology of the Fishes of the Families
Osteoglossidae, Pantodontidae, and Phractolaemidae" (Jour. Linn. Soc.,
vol. XXIX) he reports a sesamoid articular present in Osteoglossum
leichardti, and Heterotis niloticus, but absent in Arapaima gigas. In
Pantodon buchholzi, and in Phractolaemus ansorgii, he does not mention
a sesamoid articular, but his material was in the form of prepared skulls
and its absence might only mean that it was lost in preparing.

In his paper "On the Skull of Gonorhynchus greyi" (Ann. Mag.
Nat. Hist., 1905) he reports a sesamoid articular present in Gonorhynchus
greyi.

Elops saurus.

The rather small sesamoid articular is attached to the ectosteal part
of the articular far in front of the endosteal part, and is somewhat
covered with Meckel's cartilage. A deep cavity, open in front, is par-
titioned off in the ectosteal part just behind the sesamoid articular by a
thin plate of bone. This is shown in Dr. Ridewood's picture (Proc. Zool.
Soc. Lond., 1904, vol. II, p. 40).

Albula vulpes.

SA t

Fig. 5. — ALBULA VULPES

an, angular; d, dentary; ecpl, ectosteal plate; epr, endosteal process; me, Meckel's
cartilage; sa, sesamoid articular; t, tendon.

THE SESAMOID ARTICULAR 15

The sesamoid articular is a very large, thick bone closely attached
to the front of the endosteal part of the articular, with its outer surface
against the ectosteal part. Its surface is almost as great as that of the
endosteal part, and has the appearance of being in as close relationship
with Meckel's cartilage. Its structure, its union with Meckel's cartilage,
and its concave lower edge, from which the cartilage issues, are all strik-
ingly like the endosteal part.

Dorosoma cepedianum.

Dr. Ridewood reported on Dorosoma erebi, and did not mention a
sesamoid articular or show one in his picture. This species has a well
developed one in front of the cup-shaped process of the endosteal part
of the articular, and partly covered by Meckel's cartilage.

Dussumieria elopsoides.

Dr. Ridewood reports the sesamoid missing in Dussumieria acuta, but
in this species there is a small one situated against the articular and partly
covered by Meckel's cartilage.

Clupea pallasii.

Dr. Ridewood reports the sesamoid articular missing in Clupea
harangulus. In this closely related species it is present, situated against
the articular just in front of the endosteal process, and covered by the
base of Meckel's cartilage.

Opisthonema oglinum.

The sesamoid articular is moderately large, and is situated on the
upper part of Meckel's cartilage and the endosteal process.

Coilia nasus.

In this form Dr. Ridewood failed to find a sesamoid articular. A
small one is present, however, in the deep notch that is formed directly in
front of the articulating surface of the quadrate, and covered by Meckel's
cartilage. The line of demarcation between the endosteal and ectosteal
parts of the articular is not evident.

l6 THE SESAMOID ARTICULAR

Pterengraulis atherinoides.

Though much larger and of more dense bone, the sesamoid articular
is essentially as in Coilia nasus.

Engraulis encrasicholus.

In this species the sesamoid articular is large and essentially as in
Pterengraulis and Coilia, being as large as in the former. Dr. Ridewood
reports it missing, it having doubtless been lost in preparing the speci-
men, for it is easily pulled away and remains attached to its tendon.

Pterothrissus gissu (Bathythrissa dorsalis).

Dr. Ridewood reports the sesamoid articular unossified in this form.
It is, however, very well developed, and is placed saddle-like over Meckel's
cartilage remote from the endosteal process. It, like Engraulis, has this
bone so loosely attached that it is liable to be removed with its tendon.

THE SALMONOID FISHES.
Salmo irideus.

A small sesamoid articular is loosely attached against the ectosteal
part of the articular remote from the endosteal part, and is partly covered
by the slender Meckel's cartilage.

Osmerus dent ex.
In this the sesamoid articular is essentially as in Salmo.

Thymallus signifer.

The sesamoid articular is moderately large, and on the upper edge
of Meckel's cartilage remote from the endosteal part of the articular.

Argyrosomus hoyi.

In this form the sesamoid articular is rather large, situated on the
upper surface of Meckel's cartilage, and in contact with both the endo-
steal and ectosteal parts of the articular.

THE SYNODONTID^E.

Synodus lucioceps.

In this the sesamoid articular is large. It is attached to the endo-
steal part of the articular at the inner side and posterior end, and arches
over Meckel's cartilage to the ectosteal part of the articular.

THE SESAMOID ARTICULAR 17

THE EELS.

In all the examples examined the dentary forms a sheath about the
articular, hiding Meckel's cartilage completely from view, and covering
the anterior part of the sesamoid articular. The sesamoid articular ten-
don is well developed.

Symbranchus marmoratus.

A moderately large sesamoid articular is situated on the articular
above the endosteal process and base of Meckel's cartilage. The tendon
from the sesamoid articular is a flat band somewhat connected with a
similar band attached more posteriorly on the articular. A great mass
of tendinous tissue gives muscle attachment to the upper, posterior pro-
cess of the dentary.

Anguilla chrysypa.

The sesamoid articular is situated on the endosteal process of the
articular, and is covered anteriorly by the inner plate of the dentary,
under which it extends forward over Meckel's cartilage.

Synaphobranchus pinnatus.

The sesamoid articular is an exceedingly slender splint of bone cov-
ering Meckel's cartilage on its upper and inner surface, directly in front
of where the cartilage issues from the endosteal process of the articular,
and extending forward into the dentary sheath.

Leptocephalus conger.

In this form the articular is anteriorly divided. The lower division
is entirely outside of the dentary, running along its outer lower edge,
while the upper division is sheathed by the dentary. A very large sesa-
moid articular is on the endosteal part of the upper fork and forms with
it the cavity from which Meckel's cartilage issues, and which is usually
confined to the endosteal process, though here the sesamoid articular
forms the greater part of it. The sheath of the dentary fits tightly about
the articular and the sesamoid articular, leaving only a minute hole into
the dentary sheath above the sesamoid articular.

Muraenesox coniceps.

The sesamoid articular is on the endosteal process of the articular,

i8

THE SESAMOID ARTICULAR

and extends into the cavity of the dentary sheath along the top of Meckel's
cartilage. The dentary sheath is wide open behind, unlike that of Lepto-
cephalus conger.

Ophichthus triserialis, Ophisurus serpens, Lycodontis castaneus.

In these forms the sesamoid articular and its relationship to the sur-
rounding elements differ in no important way from that of Muraenesox.

Fig. 6.— LYCODONTIS CASTANEUS
d, dentary; epl, ectosteal plate; epr, endosteal process; sa, sesamoid articular.

THE HETEROGNATHOUS FISHES.
Hoplias microlepis.

The sesamoid articular is a thick, irregular bone on top of Meckel's
cartilage, and attached against the articular just anterior to the endo-
steal process of that bone. The tendon is slender.

Curimatus isognathus.

In this the sesamoid articular is placed as in Hoplias, but the bone
is thinner and flatter and fits more broadly on the articular.

Hydrocynus cuvieri.

The sesamoid is a flat bone with a tubercle for the attachment of the
tendon. It lies closely against the ectosteal part of the articular on top
of Meckel's cartilage and the endosteal process.

THE SESAMOID ARTICULAR

THE EVENTOGNATHOUS FISHES.

Deltistes luxatus, here described, but Ictiobus bubalus, Chasmistes
brevirostris, Myxostoma aureolum differ only as noted.

Meckel's cartilage appears as a short, deep, thin, somewhat quad-
rangular plate, situated rather high on the mandible. Posteriorly it
abruptly ends against the endosteal process of the articular. Anteriorly it
ends in a similar way against a portion of the dentary, which may repre-
sent the mento-meckelian ossicle of some of the Ganoids here incorporated
with the rest of the dentary.

Fig. 7. — CHASMISTES BREVIROSTRIS

an, angular; d, dentary; epr, endosteal process; me, Meckel's cartilage;
sa, sesamoid articular.

The sesamoid articular is a very large flat plate, somewhat triangular
in shape, its upper edge nearly even with the upper edge of the
mandible, and its lower about equally overlying Meckel's cartilage and
the endosteal process of the articular. It lies broadly, and is connected
rather firmly, against the articular, and its anterior end overlaps the den-
tary. This is unique to my knowledge, for in other forms it is confined
to the articular, though in the eels the dentary overlies it. In Chasmistes
the condition is the same, but in the other two forms the sesamoid articu-
lar does not reach to the dentary. Considerable variation was found in
this character (all specimens were of rather large size), and as it is of
secondary development it is of little importance. The tendon is large and
flat, and is attached to the middle of the upper edge of the sesamoid
articular.

20

THE SESAMOID ARTICULAR

Cyprinus carpio, Leuciscus bicolor, Semotilus atromaculatus, Ptycho-
cheilus oregonensis, Orthodon microlepidotus, Pogonichthys
macrolepidotus.

The sesamoid articular is more slender than in Deltistes et. al., more
closely associated with Meckel's cartilage, which is much more slender,
and it is lower in position, being remote from the upper margin of the

an

Fig. 8. — LEUCISCUS BICOLOR

\, angular; d, dentary; epr, endosteal process; me, Meckel's cartilage;
sa, sesamoid articular.

articular. It is a large flat bone lying closely against the articular, and
articulated along its lower edge with the endosteal process and with
Meckel's cartilage, being concave to fit over them. There is a prominent
tubercle of bone above its middle to which a stout tendon is attached.

Misgurnus anguillicaudus.

In this form I fail to find a separate sesamoid articular, though a
tendon is attached to the middle of the articular about in the typical place
for one. The inner part of the articular is broken up into a lace-like plate
which seems to be an integral part of it.

THE SILUROIDS.
Silurus glanis.

The sesamoid articular is smaller than in the other siluroids examined.
It is situated on the upper inner edge of Meckel's cartilage, and a small

THE SESAMOID ARTICULAR

21

process from it joins the inner surface of the ectosteal plate of the articu-
lar, but does not articulate with the endosteal process. The tendon is
large.

Amiurus catus.

A very large sesamoid articular is present. It is fitted saddle-like
around the upper part of Meckel's cartilage. Its inner edge is articulated
by a dentate suture with the inner edge of the endosteal process of the
articular which covers the posterior part of Meckel's cartilage. Its outer
surface lies flat against the inner surface of the ectosteal plate. It thus
forms the roof of a cavity for Meckel's cartilage, the inner side wall of
which is formed by the endosteal part of the articular.

Platystomatichthys sturio.

The sesamoid articular overlies the upper edge of Meckel's cartilage,
and is articulated with the endosteal process but does not extend to the
ectosteal plate of the articular. The tendon is attached to the extreme
posterior upper corner of it.

Brachyplatystoma vaillanti.

Meckel's cartilage turns upward to the point of the coronoid process.
A large sesamoid articular overlies the cartilage at a point where it
turns up from the typical horizontal portion. It is articulated with the
endosteal process of the articular, and arching over the cartilage attaches
itself to the inner face of the ectosteal plate. Posteriorly, a little above
its middle, is a blunt tubercle of bone to which the tendon is attached.

Fig. 9. — BRACHYPLATYSTOMA VAILLANTI

d, dentary; ecpl, ectosteal plate; epr, endosteal process; me, Meckel's cartilage;

sa, sesamoid articular.

22

THE SESAMOID ARTICULAR

Trachycorystes galeatus.

The sesamoid articular extends over the top of Meckel's cartilage,
connecting the endosteal process and the ectosteal plate. The tendon is
very large and stout.

Ictalurus furcatus.

In this the sesamoid articular is as in Brachyplatystoma, but much
smaller and more slender.

Selenaspis dowii, Sciadeichthys troscheli.

In these the sesamoid articular is essentially as in Amiurus.

THE GYMNOTID FISHES.
Electrophorus electricus.

The sesamoid is here long and rod-like, and is situated against the
ectosteal part of the articular in front of the endosteal part. It lies on
and slightly arches over Meckel's cartilage. A large tubercle on its
posterior end gives attachment to a very stout tendon.

Gymnotus carapo.

The sesamoid articular is a very large flat bone situated above the
base of Meckel's cartilage and the endosteal process. It projects broadly
over the inner surface of the latter and is strongly articulated to it. To
a prominent tubercle on its posterior end a large tendon is attached.

Fig. 10. — GYMNOTUS CARAPO

an, angular; d, dentary; cpr, endosteal process; me, Meckel's cartilage
sa, sesamoid articular ; t, tendon.

THE SESAMOID ARTICULAR 23

THE SCYPHOPHORI.
Gymnarchus niloticus.

Dr. Ridewood presents a good plate of the mandible of this form
(Linn. Soc. Jour., XXIX, pi. 23) in which the sesamoid articular is
shown to be nearly as large as the endosteal articular, with which it is
connected by a deeply dentate suture. It lies against the ectosteal part
of the articular.

In the above paper a sesamoid articular is reported to be present in
Mormyrops deliciosus, but not present in Pterocephalus bane.

THE HAPLOMI AND XENOMI.
Umbra lima, Dallia pectoralis.

These two are similar in having the sesamoid articular very large,
attached far forward on the ectosteal plate with which it is broadly con-
nected, concave on its lower surface to fit over Meckel's cartilage, and
far remote from the endosteal part of the articular, which is little larger
than is necessary to form the condyle for the quadrate.

Esox reticulatus.

The sesamoid articular is much smaller than in Umbra, and not
placed so far forward on the ectosteal plate of the articular, nor so
broadly connected. Otherwise it and the condition of the small endo-
steal part of the articular are similar.

Fundulus similis.

The large sesamoid articular lies about equally over the top of the
endosteal process and Meckel's cartilage, with its outer surface against
the ectosteal plate. The endosteal part of the articular, unlike that of
Esox, is ossified well forward.

THE SYNENTOGNATHI
Tylosurus marinus, T. acus.

The sesamoid articular is an exceedingly large, flat bone, suturally
attached to the ectosteal plate of the articular, and at its lower edge to

24 THE SESAMOID ARTICULAR

the endosteal process, extending forward over the posterior end of
Meckel's cartilage. Upward and backward it projects above the articu-
lar in a process that shows to a considerable extent when viewing the
mandible from the outer surface. To it the muscle is closely attached
by tendinous tissue, as herein described under adductor muscles. It is
of such a size and position that it is little wonder Cope called it the coro-
noid, though it is considerably behind the coronoid bone of reptiles, or
the coronoid cartilage of Amiatus. The anterior upper angle of the
articular of Tylosurus, which is just behind the dentary, is in the coro-
noid region.

Fig. ii. — TYLOSURUS ACUS

an, angular; d, dentary; ecpl, ectosteal plate; me, Meckel's cartilage;
sa, sesamoid articular.

Exocoetus californicus.

The sesamoid articular is here as it is in Tylosunts, except that it
scarcely extends above the articular to be visible from the outside.

Chirodorus atherinoides, Hemiramphus richardi.

These forms have the sesamoid articular essentially alike and almost
identical with that of Tylosurus.

THE HEMIBRANCHIATE FISHES.
Gasterosteus cataphractus.

In this form the sesamoid articular is 'a nodule of bone on the upper
part of Meckel's cartilage attached against the ectosteal plate of the
articular far from the coronoid region, or the upper margin of the man-
dible. It is smaller than in the other Hemibranchiate fishes.

THE SESAMOID ARTICULAR

Macrorhamphosus sagifue.

At the upper edge of the broad Meckel's cartilage is a rather wide,
disk-like sesamoid articular placed flat against the ectosteal plate, con-
siderably above the endosteal part of the articular, and directly below the
ascending process of the dentary. It extends upwards more than usual,
thus approaching the condition of the Synentognathous fishes.

Fistularia petimba.

The dentary and articular are very complex in shape. Each of them
has three forks, which more or less interlock. The middle fork of the
dentary (by far the largest) fits between the middle and lower forks of
the articular in a deep notch that extends back nearly to the condyle of
the quadrate, thus almost dividing the articular into two parts. Directly
above this division in the articular are the endosteal process of the articu-
lar and Meckel's cartilage. The upper forks of both elements arch up-
ward, leaving an open space, above which they nearly meet. It is to
the posterior edge of this upper fork of the articular, to a small process
of bone, that the adductor tendon is attached. The process is remote from
Meckel's cartilage, and is probably not the homolog of the sesamoid
articular. In the smallest specimen available (5 inches long) no suture
was apparent to indicate a separate element in this region.

Aulostoma valentini.

The elements of the mandible differ from those of Fistularia only
in unimportant details. The upper fork of the dentary is scarcely devel-
oped, and that of the articular does not arch over an open space, but the
posterior part of it bears a process (larger than in Fistularia) to which
the adductor tendon is attached.

Aulorhynchus flavidus, Aulichthys japonicus.

The sesamoid articular is a very large plate, lying flat against the
ectosteal part of the articular, and extending up from Meckel's cartilage,
which it only barely touches, towards the coronoid region. To its upper
end is attached a strong tendon, and to the coronoid process is attached
a smaller, more fibrous one.

26 THE SESAMOID ARTICULAR

THE SALMOPEROffi.
Percopsis guttatus.

The sesamoid articular is very loosely attached to the upper surface
of Meckel's cartilage, slightly in contact with the endosteal process of
the articular, but scarcely with the ectosteal part.

Aphredoderus sayanus.

The sesamoid articular is situated against the ectosteal plate of the
articular, above and outward from the endosteal process, and nearly
covered from sight by Meckel's cartilage.

THE PERCESOCES.
Atherinopsis californiensis

In this the sesamoid articular is very long. In a mandible 22 mm.
in length it is one-third of this length. It is closely attached to the ecto-
steal plate of the articular, and still more closely to the top of the endo-
steal process. It overlies the top of Meckel's cartilage, and projects to
a considerable extent between it and the articular. The adductor tendon
attached to it is large.

Menidia notata.

The sesamoid articular is short and deep, attached to the upper and
inner surface of Meckel's cartilage and the endosteal process of the articu-
lar, and to the ectosteal plate above them.

Sphyraena argentea.

The sesamoid articular is a small sliver of bone attached to the ecto-
steal plate of the articular in front of the endosteal process. It is nearly
covered by Meckel's cartilage.

Mugil cephalus.

The sesamoid articular is here a rather large, thick disk of bone
attached to the ectosteal plate and to the top of the endosteal process of
the articular. It extends considerably above Meckel's cartilage, and is
covered but slightly by it.

THE SESAMOID ARTICULAR 27

THE RHEGNOPTERI.

Polydactylus approximans.

In this form the endosteal process stands away from the ectosteal
plate (inward from it), and a flat bridge of bone projects upward from
it to near the edge of the ectosteal plate just in front of the condyle for
the quadrate. The cavity behind it (outward from it) is doubtless the
homolog of the chamber in the mandible of Elops in the same region,
where it is completely separated as a chamber rather than simply bridged
across. As the partition in Elops unquestionably belongs to the ectosteal
part of the articular, it probably does here, though it is so completely
anchylosed with the endosteal process that its boundary is obscure. The
sesamoid articular is situated in the space between the endosteal process
and the ectosteal plate. Its middle is opposite the end of the process, and
it is equally in contact with the process and Meckel's cartilage, which
entirely hide it as the mandible is viewed from the side.

THE BERYCOID FISHES.

Beryx splendens, Hoplostethus japonicus, Monocentris japonicus,

Polymixia japonica.

In these forms the sesamoid articular is against the ectosteal plate
on the outer side, and the endosteal process and Meckel's cartilage on the
inner. Viewed from the side, the process and cartilage hide it from view.
In Hoplostethus it is rather flat and disk-like. In Beryx it is a thick,
elongate splint of bone with a tubercle at its middle for the attachment of
the tendon. In Polymixia and Monocentris it is very small but thick.
In Monocentris it is very loosely attached.

Holocentrus ascencionis, Myripristis occidentalis.

These two forms are similar in the condition of the sesamoid articu-
lar. It is of moderate size, situated against the ectosteal plate just in
front of the endosteal process, and on its lower edge folding over the top
of Meckel's cartilage.

THE SCOMBRID^E.

Scomber japonicus, Scomberomorus sierra, Sarda chilensis, Gymno-
sarda palamis, Acanthocybium solandri.

In all of these the sesamoid articular is rather small, situated on

28 THE SESAMOID ARTICULAR

Meckel's cartilage and against the ectosteal plate of the articular. In
Scomber it is on the upper surface of the cartilage. In Scomberomorus
it is somewhat between the cartilage and the articular. In Sarda, Gym-
no sar da and Acanthocybium it likewise is between but more above the
cartilage. In all but Acanthocybium it is in contact with the end of the
endosteal process, but in none of them is it on the upper surface of the
process.

THE CARANGID^:.

Selene oerstedii, Naucrates ductor, Lichia glauca, Alectis ciliaris,
Citula dorsalis, Chloroscombrus chrysurus, Caranx pisquetus,
Megalaspis cordyla.

The sesamoid articular is covered more or less completely by the
endosteal process and Meckel's cartilage as viewed from the side, lying
with its outer surface against the ectosteal plate.

Trachurops crumenopthalmus, Nematistius pectoralis, Elegatis bipin-
nulatus, Decapterus punctatus, Trachurus mediterraneus.

The sesamoid articular is as in Selene et. al. but more anteriorly
placed, and very slightly if at all in contact with the endosteal process.
In Trachurus it is thinner and more broadly connected with the ectosteal
plate than in the others.

Vomer setipinnis, Scomberoides toloparah, Gnathanodon speciosus.

In these the sesamoid articular is more on top of the endosteal
process and Meckel's cartilage than behind them. In Vomer it is much
less broadly connected with the ectosteal plate than in the other two.

OTHER SCOMBROID FISHES.
Leiognathus fasciatus.

The lower part of the articular is folded inward as well as abruptly
thickened, thus forming a wide longitudinal shelf. The endosteal process
is obscure. The sesamoid articular is a wide, thin plate (5 mm. long in a
mandible 27mm. long), closely attached horizontally to the longitudinal
shelf. Meckel's cartilage is a very thin rod which disappears under the
sesamoid articular. To the upper, flat surface of the sesamoid articular
two equally stout tendons are attached: one a little behind its middle,

THE SESAMOID ARTICULAR 29

and one to its posterior end. Opposite the base of each tendon there
is a rather deep depression in the articular, which show on the outer
surface of the mandible as very thin spots where the bone is easily broken
through. I have found no other example where two tendons join the
sesamoid articular. The relationship of the tendons is herein described
in the section devoted to adductor muscles.

Promethichthys prometheus, Lepidopus caudatus, Trichiurus lepturus.

The sesamoid articular is similar to that in the fishes of the family
Scombridae, rather small, more or less between the ectosteal plate and
Meckel's cartilage, and just at the anterior end of the endosteal plate,
with which it is in contact except in Lepidopus.

Pomatomus saltatrix, Palometa triacanthus, Peprilus paru.

The sesamoid articular is on the upper surface of Meckel's cartilage
and the endosteal process, and on its outer surface it is against the ecto-
steal plate. In Rachycentron it is rather large and only slightly in con-
tact with the endosteal process.

THE PERCOID FISHES.

Lobotes pacificus, Pomoxys sparoides, Priacanthus catalufa, Apogon
maculatus, Sciaena stellifera, Nebris occidentalis, Mentacirrhus
alburus, Polycirrhus dumerili.

In these the sesamoid articular is moderate in size or small, and is
situated against the ectosteal plate with the endosteal process and Meckel's
cartilage overlying it and hiding it from view more or less as it is viewed
from the side.

Dentex vulgaris, Sargus rondeletii, Archosargus probatocephalus,
Pagrus vulgaris, Diplodus argyrops, Pagrellus erythrynus,
Lutianus aya, Girella nigricans, Oblata melanura.

The sesamoid articular is very much as in Lobotes et. al., but is on
top of the endosteal process and Meckel's cartilage, and very little, if at
all, hidden behind them. In Archosargus and Pagrus it is much more
over the endosteal process than over Meckel's cartilage. In Diplodus
this condition is reversed. In the others it is about equally over both.

THE SESAMOID ARTICULAR

Micropterus salmoides, Labrax lupus, Serranus atrarius, Roccus
lineatus, Cynoscion maculatus.

In these the sesamoid articular is as in Lobotes et. al., but more
anterior in position and not in contact with the endosteal process.

S4 epr

Fig. 12. — LABRAX LUPUS

an, angular; ecpl, ectosteal plate; epr, endosteal process; me, Meckel's cartilage;
sa, sesamoid articular ; t, tendon.

Orthopristis chrysopterus.

In this the sesamoid articular is very large. It is closely attached
to the ectosteal plate, with about a sixth of its length resting on the endo-
steal process and the rest on Meckel's cartilage. It is long and tapers
to a point anteriorly, while on the top of its posterior part is a very large
tubercle of bone for the attachment of the adductor tendon.

THE PHARYNGOGNATHI.
Tautoga onitis, Crenilabrus pavo.

The sesamoid articular is on top of Meckel's cartilage, attached
against the ectosteal plate and slightly attached to the endosteal process.

Harpe rufa.

There are two adductor tendons originating on the same muscle
that are similar in size and structure as in Leiognathus, but only the
upper one joins the sesamoid articular. The other is attached to the
articular ventral to the endosteal process. When stripped of their muscle

THE SESAMOID ARTICULAR 3!

they appear at the upper end as a single flat band, but they are easily
separated. The sesamoid articular is as in Tautoga, though more broadly
attached to the endosteal process.

Sparisoma flavescens.

The sesamoid articular is rather flat and disk-like. It is firmly at-
tached against the ectosteal plate and the upper side of the endosteal
process and Meckel's cartilage.

THE SQUAMIPINNES.

Chaetodipterus faber, Chaetodon bimaculatus, Holocanthus tricolor,

Platax teira.

The sesamoid articular is closely attached against the ectosteal plate
and about half hidden by the overlying endosteal process. In Chaetodip-
terus it is only slightly in contact with Meckel's cartilage ; in the others
it is broadly in contact. In Chaetodon it is more above the process and
cartilage.

Drepane punctata.

In this the endosteal process is obscure. The sesamoid articular is
small and disk-like, and is closely attached against the ectosteal plate,
where it is hidden behind Meckel's cartilage.

Zanclus canescens.

The sesamoid articular is attached against the ectosteal plate only
very slightly in contact with Meckel's cartilage and remote from the
endosteal process.

Teuthis bahamus.

The articular is very short and its endosteal process reaches to its
anterior end, leaving but little space for Meckel's cartilage. Almost
directly at the anterior end of the articular, on the upper surface of the
endosteal process and the base of Meckel's cartilage, is a rather large
sesamoid articular.

Xesurus punctatus.

In this the endosteal process does not occupy so great a part of the
articular as in Teuthis, and the sesamoid articular is confined to the upper
surface of Meckel's cartilage and the endosteal part of the articular.

THE SESAMOID ARTICULAR

THE AMPHACANTHI.

Siganus fuscescens

i

The sesamoid articular is broad and flat; its flat side is firmly at-
tached against the ectosteal plate, and its lower edge rests on the endo-
steal process and Meckel's cartilage. The tendon is attached near its
center.

THE PLECTOGNATHI.
Balistes vetula.

The sesamoid articular is a thick nodule of bone on the upper sur-
face of the endosteal process (whose limits are obscure) and Meckel's
cartilage.

Spheroides annulatus.

In this form the sesamoid articular more evidently originates within
the tendon than in any other example I have encountered. Near the
mandible end of the tendon an area of it is ossified, forming a shaft of
bone about four times as long as broad. The tendon is attached to the
region of the endosteal process, which is obscure, at some distance behind
Meckel's cartilage. Between the mandible and the ossified portion of the
tendon (sesamoid articular) is a considerable area of unossified tendon.
A large part of the inner surface of the dentary as well as the upper angle
(coronoid region) of the articular and dentary, is covered by a great
mass of tendon from the adductor muscles. The tendon that forms the
sesamoid articular ossification is below this mass of tendon and compara-
tively small.

Fig. 13. — SPHEROIDES ANNULATUS
a, articular; d, dentary; sa, sesamoid articular.

THE SESAMOID ARTICULAR 33

Ovoides setosus.

This differs from Spheroides only in having the sesamoid articular
attached more directly to the mandible without the intervention of much
tendinous tissue, and in the bone being enlarged and spreading out at the
end of the tendon.

Chilomycterus schoepfi.

The enclosteal process is obscure. The sesamoid articular is rather
large, and is attached closely against the articular just above Meckel's
cartilage, which covers its lower edge.

Lactophrys tricornis.

The sesamoid articular is situated on the surface of the endosteal
process, and on Meckel's cartilage to a very small extent.

THE MAIL-CHEEKED FISHES.

Scorpaena senorae, Scorpaena scrofa, Scorpaena ustulata, Scorpaena
plumieri, Sebastes marinus, Sebastolobus alaskanus, Sebastodes
flavidus, Prionotus evolans, Prionotus horrens, Trigla corax.

The sesamoid articular is situated against the ectosteal plate just
in front of the endosteal process, and more or less completely hidden by
Meckel's cartilage. In Scorpaena scrofa it is rather large and runs for-
ward in a splint of bone. In Trigla it is moderate in size, and in the
others it is small.

Zaniolepis latipinnis, Ophiodon elongatus, Liparis agassizii.

These differ from Scorpaena et. al. in having the sesamoid articular
covered by the endosteal process as well as by Meckel's cartilage. In
Liparis it is only slightly behind the process.

Hexagrammos decagrammus, Scorpaenichthys marmoratus, Myoxo-
cephalus octodecemspinosus, Podothecus acipenserinus.

The sesamoid articular is more on top of the endosteal process and
Meckel's cartilage than covered by them. It is closely connected with
the ectosteal plate.

34 THE SESAMOID ARTICULAR

Hemilepidotus jordani.

The sesamoid articular is on top of Meckel's cartilage against the
ectosteal plate and remote from the endosteal process.

THE GOBIES.
Dormitator latifrons.

The sesamoid articular is large and disk-shaped, with its flat side
attached firmly to the ectosteal plate, its posterior lower edge to the endo-
steal process, and its lower edge extending downwards over Meckel's
cartilage to a considerable extent. The tendon is attached near its middle.

Philypnus dormitator.

The sesamoid articular is a rather small triangular bone, situated on
top of Meckel's cartilage well in front of the endosteal process, and
slightly against the ectosteal plate.

THE DISCOCEPHALI.
Echenis naucrates.

The endosteal process is obscure. The sesamoid articular is a large,
flat bone of rather complex outline, attached against the articular just
above and slightly covered by Meckel's cartilage.

THE TRICHODONTID^E.
Trichodon trichodon.

The sesamoid articular lies just in front of the endosteal process
between Meckel's cartilage and the ectosteal plate.

THE LATILID^E.

Lopholatilus chamoeleonticeps.

The sesamoid articular is a rather large, thick bone wedged between
the ectosteal plate and the endosteal process.

THE SESAMOID ARTICULAR 35

THE BATHYMASTERIOffi.

Bathymaster signatus.
The sesamoid articular is here essentially as in Trichodon.

THE URANOSCOPIDJE AND TRACHINID^.
Uranoscopus scaber, Trachinus radiatus.

The large sesamoid articular is broadly attached against the ecto-
steal plate with its lower surface resting on the endosteal process and
Meckel's cartilage.

THE BLENNIOID FISHES.
Lumpenus anguillaris, Cebedichthys violaceus, Ziphidion mucosum.

The sesamoid articular is large and attached closely to the ectosteal
plate, resting at its lower edge on Meckel's cartilage, and sometimes on
the endosteal process ; in Lumpenus it rests equally on the cartilage and
process ; in Cebedichthys it is only slightly in contact with the process ;
in Ziphidion it is remote from the process.

Heterostichus rostratus.

In this the sesamoid articular is small and against the ectosteal plate
remote from the endosteal process, while Meckel's cartilage covers its
inner surface.

Anarrichthys ocellatus.

This has the sesamoid articular situated as in Lumpenus, but it is
much larger, roughly elliptical in outline, and very thick, with its thick-
ness extending to its edges.

THE ZOARCIDJE.
Lycodopsis pacificus, Lycodes brevipes.

The sesamoid articular is long and rather thick. It is wedged in
between the ectosteal plate and Meckel's cartilage, extending up, however,
considerably above the latter. Its posterior end extends a little behind
the anterior end of the endosteal process.

3D THE SESAMOID ARTICULAR

THE ANACANTHINI.
Melanogrammus aeglifinus, Brosme brosme.

The sesamoid articular is large and is attached against the ectosteal
plate just at the end of the endosteal process. It extends downward
somewhat behind Meckel's cartilage, but its bulk is above the cartilage
and arches over its upper surface.

Microgadus tomcod.

The condition of the sesamoid articular is most surprising, for it
is apparent that part of it at least is endosteal in origin, ossifying directly
from Meckel's cartilage. For a short distance in front of the endosteal
process Meckel's cartilage is as usual, then it is abruptly ossified in a rod
of bone in front of which, as it runs toward the dentary, it is again carti-
laginous. The posterior outer part of the ossification is enlarged against
the ectosteal plate, and to this enlargement the stout adductor tendon is
attached. It appears evident that the enlarged outer portion is the usual
sesamoid articular, and that Meckel's cartilage in ossifying has become
anchylosed with it. The part anterior to the enlarged portion is a white,
poorly organized, granular bone, but at the enlargement it is clear and
dense. Several speciments between 8 and 10 inches in length were exam-
ined, and considerable variation found in the length of the ossified por-
tion of Meckel's cartilage.

Fig. 14. — MICROGADUS TOMCOD

an, angular ; d, dentary ; ecpl, ectosteal plate ; epr, endosteal process ; me, Meckel's
cartilage; sa, sesamoid articular.

THE SESAMOID ARTICULAR

37

Gadus callarias, Pollachius brandti.

A small part of Meckel's cartilage is replaced by the sesamoid articu-
lar and the cartilage is unossified before and behind it. The outer part
of the ossification spreads out in a broad plate of bone over the ectosteal
plate, where it is firmly attached. This outer part resembles the sesamoid
articular as found in many forms. A stout tendon is attached to a rather
wide base formed by both the part replacing cartilage and the part against
the ectosteal plate. The endosteal process is ossified on the surface only,
for a considerable space at its anterior end leaving cartilage within.

epr

Fig. 15. — GADUS CALLARIAS

an, angular; d, dentary; ecpl, ectosteal plate; epr, endosteal process ; me, Meckel's
cartilage; sa, sesamoid articular.

THE HETEROSOMATA.

Psettichthys melanostictus, Eopsetta jordani, Parophrys vetulus,
Citharichthys sordidus.

The sesamoid articular is against the ectosteal plate directly in front
of the endosteal process, and covered more or less by Meckel's cartilage.
In Parophrys it is larger than in the others and more disk-like.

Usinosita japonica.

The endosteal process is scarcely developed, and the endosteal part
of the articular consists of little more than enough bone to form the
concavity for the articulation of the condyle of the quadrate. Articulated
to this bone directly in front of the concavity is a large sesamoid articular
which rests broadly on Meckel's cartilage.

38 THE SESAMOID ARTICULAR

THE PEDICULATI.

Antennarius sp.

The small sesamoid articular rests against the ectosteal plate directly
in front of the endosteal process, and is nearly covered from sight by
Meckel's cartilage.

Ogcocephalus radiatus.

The sesamoid articular is broad and disk-like, and is attached tightly
against the ectosteal plate and the upper surface of the endosteal process,
while anteriorly it rests on the surface of Meckel's cartilage.

SUMMARY

Specimens from nearly every important group of fishes have been
examined and are herein reported upon. The sesamoid articular was
found in every group that is more highly specialized than the primitive
Clupeoid fishes as well as in one of the Ganoids, Amiatus, though here its
identity may be open to some doubt.

The sesamoid articular is variable in size and somewhat in position,
though it is always in relationship with Meckel's cartilage — usually in
very close association with it. Typically, as in the perch-like fishes, it
is rather small and more or less covered by Meckel's cartilage. Some-
times, as in Albula, it is as large as the endosteal process ; or, as in some
of the Synentognathous fishes, it is so large that it projects above its sur-
rounding elements, so that it is visible from the outer surface. Sometimes
it is loosely attached to the mandible and is easily pulled away with its
tendon. Sometimes it is attached to the mandible by a dentate suture and
is disarticulated with difficulty. Usually it is more or less broadly at-
tached to the ectosteal plate of the articular, but sometimes only narrowly
attached, and occasionally scarcely, or not at all, in contact. Sometimes
it lies saddle-like over the top of Meckel's cartilage, remote from any bone.
It is variable in its connection with the endosteal process, but as the at-
tachment may depend upon the extent of the ossification of Meckel's
cartilage to form the process, this is without significance. In at least
one case (Lew gnathus) two equally stout, dense tendons join the sesa-
moid articular at a considerable distance from each other.

The sesamoid articular is doubtless, as its name implies, a sesamoid
bone. This is especially well shown in Spheroides annulatus, where the
adductor tendon has obviously ossified for a short space, leaving an
interval of tendon between the ossified portion and the mandible. Inter-
mediate between this condition and the more typical condition is the sesa-
moid articular of Ovoides, a close relative of Spheroides, where the inter-
val of cartilage between the tendon bone and the mandible has disappeared
and the former has spread out to some degree on the mandible.

On the other hand Microgadus and Gadus appear to have the bone
partly endosteal in origin, or at least a bone endosteal in origin has be-
come anchylosed with it. In Gadus, however, it is not altogether clear

4O THE SESAMOID ARTICULAR

that the sesamoid articular in replacing a section of Meckel's cartilage
is necessarily the ossified cartilage. The sesamoid articular may have
surrounded Meckel's cartilage, and developing from without inward may
thus have replaced it. The beginning of such a process is suggested by
Melanogrammus, and other forms, where the sesamoid articular arches
over the upper surface of Meckel's cartilage. Such, however, does not
account for the condition in Micro gadus (M. tomcod), where the carti-
lage is ossified in a rod in front of the sesamoid articular.

The fact that ossification has taken place in Meckel's cartilage sug-
gests an apropos observation of Dr. Ridewood's (Proc. Zool. Soc. Lond.,
1904, p. 56) in regard to bones that are both ectosteal and endosteal in
origin, whether the two sorts are anchylosed or distinct from each other.
"The endosteal ossification has been set up in sympathy with the ossi-
fication taking place in the dermal tissues. The process of ossification is
infectious, if one may employ such a term in this connection."

It is significant that in every case the sesamoid articular is in close
relationship with Meckel's cartilage (or with the endosteal process, which
is ontologically the same). Even in cases where it has developed
upwards towards the coronoid region it still retains its connection with
the primitive cartilage.

Moreover, in some cases, as Albula, it is strikingly like the endosteal
part of the articular, and appears (at least superficially) to bear the same
relationship with Meckel's cartilage — the cartilage apparently issuing
from it as it does from the endosteal process.

Other tendons than that to the sesamoid articular join the mandible
at various places, sometimes originating on the same muscle and same
place as does that to the sesamoid articular; but they are nearly always
less compactly organized, though often they are larger. There is never
a second sesamoid articular developed for these other tendons, though
they may appear to be more important in doing the greater share of the
work.

The sesamoid articular is of no use in taxonomy of groups larger
than species, as it often differs within the genus. To pronounce upon
its value in differentiating species, or upon its individual variation, would
necessitate much more work within narrower limits than has been here
undertaken.

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