Samuel Wendell Williston.

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Triassic times, after long existence as a mere vestige. The posterior
of the two coracoids also disappeared in late Triassic times, in rep-
tiles at least, though a vestige may possibly be present in our own
shoulder girdle. The scapula, clavicles, and anterior one of the two
coracoids, the so-called procoracoid, are still present in most reptiles;
in snakes only are they wholly absent, though much reduced and
non-functional in some lizards.

Clavicular Girdle

The clavicular girdle is variable among the temnospondyl am-
phibians, dependent, as in reptiles and higher vertebrates, upon the
habits of the animals. In the aquatic types of all Stegocephalia the
clavicles and interclavicles are rugose [on the ventral side], heavy
and broad, forming more or less of a pectoral buckler — a peculiar
adaptation to their water habits, perhaps in a measure analogous to
the plastron of the turtles or the extraordinary development of the
coracoids in the plesiosaurs. In such forms also, the cleithrum is re-
duced. The girdle in the adult land forms, of which Eryops (Fig. 108)
and Cacops (Fig. 96 a) may be taken as types, is almost indistinguish-
able from that of their contemporary cotylosaurs, except that the
cleithrum is larger and the interclavicle less elongate. They are
smooth throughout in Cacops, the more terrestrial form.

Cleithrum. The cleithrum so generally characteristic of the Stego-
cephalia (Figs. 96 A, 108) was doubtfully ever functional in reptiles,

1 [According to Watson, the coracoid originally was a single piece which never be-
came subdivided in the amphibians, cotylosaurs, or ordinary reptiles, the subdivision
occurring only in the Theromorpha, Therapsida, and mammals. — Ed.]



THE PECTORAL AND PELVIC GIRDLES



127



whatever may have been its function in the amphibians ; and it was
never large. It is known only in certain members of the Cotylo-
sauria, Theromorpha, Dinocephalia, and Anomodontia, best devel-
oped perhaps of all in Diadectes and its alHes of the Cotylosauria
(Fig. 95), where its somewhat spatulate upper extremity partly
overlies the front, upper border of the scapula, articulating below
with the stem of the clavicle. It is vestigial in some forms and seems
to be quite wanting in others. Among the Theromorpha it has been
observed in Edaphosaurus (Fig. 98) as a rod-like bone at the upper




Fig. 97. Clavicles and interclavicle of Ophiacodon
(Theromorpha).



front border of the scapula. In the Anomodontia and Dinocephalia
(Fig. 107 d) it is a feeble splint, clearly a vestige. There have been
several theories as to what has become of it, but none is demon-
strable. Its vestigial condition in various cotylosaurs indicates its
entire disappearance.

Clavicles. Clavicles are usually present in reptiles. They are ab-
sent in the Crocodilia, serpents, Mosasauria, and some Sauria; more
or less vestigial in some lizards ; and either absent or vestigial in the
Pterosauria and Dinosauria.

In crawling reptiles (Figs. 96 B-99) they are usually curved bones,
with a dilated mesial extremity, articulating on the ventral side of



128



THE OSTEOLOGY OF THE REPTILES



the end of the interclavicle ; and a more or less slender stem which
articulates with the front border of the scapula, or its acromion when
present, and also with the lower end of the cleithrum when that bone
is present. In modern lizards the clavicles articulate usually with the
front border of the cartilaginous suprascapula (Fig. 99). The inner
end in some lizards is broad and perforated (Fig. 99 c).




Fig. 98. Edaphosaurus novomexicanus (Theromorpha). Pectoral girdle,
two fifths natural size: c, cleithrum; cl, clavicle; sc, scapula.



The clavicles of the Chelonia are known as the epiplastra of the
plastron (Fig. 100). In the Nothosauria (Fig. loi) they are normal
but very stout, firmly united with the scapula and with each other.
The clavicles of the Plesiosauria (Fig. 102) are remarkable in some
respects. Usually they are a pair of thin, triangular bones, lying
upon the inner or visceral surface of the proscapular process of the
scapula (corresponding to an acromion), of the interclavicle and
sometimes also of an anterior process from the coracoid ; they may
be absent. In the Ichthyosauria (Fig. 103), they are slender, some-
times coossified with each other; nor are they expanded mesially in
either the Phytosauria or Choristodera (Fig. 104), and all water rep-
tiles. Doubtful vestiges of the clavicles have been reported in the
pterodactyls.




Fig. 99. Pectoral girdles (Lacertilia): A, B, Iguana; C, Zonosaurus (after Siebenrock).

Natural size.




Fig. 100. Primitive chelonian pectoral girdle: Stegochelys.
After Jaekel.

129




Fig. ioi. Pectoral girdle of Nothosaurus (Nothosauria), from photograph by
E. Fraas: id, interclavicle; cl, clavicle; sc, scapula; cor, coracoid.




Fig. 102. Pectoral girdle of Trinacromerum (Plesiosauria), from above: ic, interclavicle;
cl, clavicle; sc, scapula; c, coracoid.



THE PECTORAL AND PELVIC GIRDLES



131




Fig. 103. Pectoral girdle of Ichthyosaur, Baptanoden {Ophthalmosaurus).
After Gilmore.




Fig. 104. Pectoral girdle of Champsosaurus (Choristodera).
After Brown.

Interclavicle. The interclavicle in the earliest-known reptiles (Fig.
96 B, c, d) is an elongate bone with a dilated but not T-shaped an-
terior extremity. The stem underlies the approxi-
mated mesial borders of the coracoids, usually ex-
tending beyond them. In a specimen referred to
Pantylus (Fig. 105), a primitive cotylosaur, the
interclavicle is forked in front and somewhat fan-
shaped behind, shaped very much like that of the
monotremes. In the later cotylosaurs the front end
is more dilated, as usual with all later reptiles. In
the known forms of the Therapsida (Fig. 107 c) the
shape is usually like that of the Theromorpha and
Cotylosauria. It is very short and fan-shaped in Fi«- i°5. Pantylus

. -^ (Cotylosauria): inter-

Lystrosaurus of the Anomodontia (Fig. 94 d), where clavicle (<v/) and cor-

acoid {cor). Natural
size.




Broom attributes its reduction to water habits.



132 THE OSTEOLOGY OF THE REPTILES

In the Chelonia it is the entoplastron (Fig. loo.) In the Croco-
diha (Fig. 121 d) and Mosasauria it is slender and free at the
anterior end. The stem is short in the Ichthyosauria (Fig. 103),
vestigial in the Nothosauria (Fig. loi). When present in the plesio-
saurs it is an oval or triangular bone, in the earlier forms imper-
forate, in the later ones with a median interclavicular notch or
foramen (Fig. 102). The interclavicle is absent in the Pterosauria,
Dinosauria, chameleon lizards, and some plesiosaurs.

Scapular Girdle

The scapular girdle, or scapulo-coracoid of the aquatic temno-
spondyl amphibians of early Permian times, like that of the aquatic
reptiles, is broad and short, but that of the terrestrial types is prac-
tically indistinguishable from the girdle of the contemporary rep-
tiles. Each side, in both the amphibians and early reptiles, is com-
posed of three bones more or less closely fused: a dorsal one, the
scapula, and two ventral ones ; the anterior one commonly called the
procoracoid; and a posterior one, often called metacoracoid. The
posterior bone was lost in all reptiles by the close of Triassic times. ^

The three bones of the land Stegocephalia (Figs. 96 a, 108) are so
firmly coossified that their sutural distinctions have rarely been ob-
served. Among the Cotylosauria (Fig. 96 b, c) the union was less
firm, or became invisible later in life; their sutural divisions have
occasionally been observed. Among the Theromorpha, the posterior
coracoid, the metacoracoid, is often found separated (Fig. 106), or
united by a loose suture; in some forms (Fig. 96 d) it remained car-
tilaginous throughout life, and in all forms it probably did not ossify
till growth was far advanced. Among most of the Therapsida the
three bones (Fig. 107 a, b, d) fuse in maturity, but not in some, if not
all, the Dinocephalia (Fig. 107 d). In the Proganosauria the divi-
sion between the two bones, if present, has never been observed. In
the Eunotosauria of the Upper Permian, the two bones are distinct.
In no other reptile has the metacoracoid been certainly observed,
though it has been affirmed in the Rhynchocephalia {Hyperoda-
pedon) , an error.

1 [For a different view of the fate of the two coracoids see Watson, 1917, Journ.
Anat., vol. Lii; Romer, 1922, Anat. Record, vol. xxrv, pp. 39-47. — Ed.]



THE PECTOR.\L AND PELVIC GIRDLES



133



It is probable that the three bones early acquired a firm union,
both ontogenetically and geologically, and that there was a progres-
sive separation and delayed ossification of the posterior bone in the




Fig. 106. Dimetrodon (Theromorpha) : scapula (sc), coracoid {cor),
and metacoracoid {mcor).

line leading toward the modern reptiles at least. It is known that in
Ophiacodon from the Permocarboniferous, ossification of the meta-
coracoid did not occur till late, and that in Varanops (Fig. 96 d) it
never ossified. This doubtless explains its absence in all known
specimens of Paleohatieria, formerly placed among the Rhyncho-
cephalia. Paleontological evidence that it is the posterior bone



134



THE OSTEOLOGY OF THE REPTH^ES



which has functionally disappeared in all modern reptiles, and not a
fusion of the two, now seems complete. The coracoid of lizards,
crocodiles, and Sphenodon is homologous with the anterior of the
two bones, the so-called procoracoid. It was Howes and Lydekker
who first reached this conclusion, and who proposed the name meta-
coracoid for the posterior bone. Whether this conclusion is the right
one so far as the monotreme mammals are concerned is still a de-




FiG. 107. Pectoral girdles (Therapsida): A, Galeops (Dromasauria). Natural size. B,
Galechirus (Dromasauria). Natural size. C, Galepus (Dromasauria). About three fourths
natural size. D, Moschops (Dinocephalia). One fifth natural size.

batable question. The two coracoids in these mammals seem, and
generally are considered to be, homologous with those of the early
reptiles. Broom has suggested that in the evolution of the mammals
the posterior bone, that is, the metacoracoid, was retained, thoug]^
lost in the reptiles. Gregory, however, has offered another solution
of the problem that would homologize the anterior or "procoracoid"
of the reptiles with the posterior bone of the Monotremata. He
thinks that three elements are involved in the problem of their evo-
lution:

"(a) The epicoracoid of Sphenodon, lizards and monotremes, a
sheet of bone lying immediately above the clavicles, and never reach-
ing the glenoid surface.

" (6) The true coracoid, or so-called procoracoid, lying behind the



THE PECTORAL AND PELVIC GIRDLES 135

clavicles, originally pierced by the coracoid foramen, primitively
forming at least the front part of the glenoid, often articulating with
the sternum.

" (c) The metacoracoid of Permian reptiles, originally forming
the back part of the glenoid region, lost in later reptiles (Williston),
and in mammals except when preserved as a vestigial element."

It is true that such an element as the epicoracoid has not been
found ossified in the early reptiles, but neither have numerous
other bones in the mesenchyme of mammals, and its ossification in
mammals would be nothing remarkable. A comparison of the epi-
coracoid of lizards (Fig. 99 b) with that of monotremes will show
their identity in relations. And doubtless a similar epicoracoid filled
in the interval between the coracoids above the clavicles and inter-
clavicles in the early reptiles (Fig. 96 d) . Should it eventually result
that Broom's theory is the correct one, that both coracoids have re-
mained in the Monotremata, the posterior one of which presumably
represents the chief ossification of the coracoid process of higher
mammals, then modern reptiles have no true coracoid, and the bone
so called must be known as the procoracoid. The author believes
that Gregory's theory is more probable. But, until the real homol-
ogies are fully determined, and to save confusion for the present, the
terms procoracoid for the anterior bone, metacoracoid for the pos-
terior, are adopted in this work.

In all known reptiles possessing a metacoracoid, the suture sepa-
rating it from the procoracoid enters the glenoid fossa (Fig. 106), ex-
cept in certain therapsids (Fig. 107), where it joins the scapular
suture a little in front of the articular surface. It passes directly in-
ward to terminate in the free border. The scapula-procoracoid
suture, in all the Cotylosauria and Theromorpha (Fig. 106) at least,
divides nearly equally the glenoid surface in front of the meta-
coracoid, and is thence directed forward and upward to terminate
in the front border.

The supracoracoid foramen, always present in the procoracoid
(Figs. 95, 96, 99, 100, 106, 107), though not in the epicoracoid of the
monotremes, and usually present in the coracoid of later reptiles
(Figs. 112, 113), is absent in the Chelonia (except the Triassic Stego-
chelys), the Pterosauria, Ichthyosauria, Plesiosauria, Rhynchosau-
ria (Howesia), many Phytosauria, and the Thalattosauria — chiefly



136



THE OSTEOLOGY OF THE REPTILES



water reptiles, it is seen. It may, in some, be represented by a notch
between the scapula and coracoid (Figs. 103, iii), doubtless its orig-
inal position.




Fig. 108. Pectoral girdle of Eryopj (Temnospondyli).
Two thirds natural size.



The scapular girdle of the terrestrial temnospondylous (Fig. 108)
amphibians has three foramina piercing it: the supracoracoid fo-
ramen, already mentioned, entering a little in front of and below the



THE PECTOR.^ AND PELVIC GIRDLES 137

glenoid fossa and opening on the inner side at the lower end of the
subscapular fossa; the glenoid foramen, entering the glenoid fossa
and opening on the inner side in front of the subscapular fossa ; and
the supraglenoid foramen entering the supraglenoid fossa near the
hind border and opening at the upper end of the subscapular fossa.
The glenoid foramen has not been observed in reptiles. The supra-
glenoid foramen is present in the Cotylosauria (Fig. 95), Thero-
morpha (Fig. 96 d), probably the Therapsida, in most modern Lacer-
tilia (Fig. 99), and in Sphenodon. It will probably be found in many
other forms when searched for. Its external orifice, however, varies
much, even in the Theromorpha. In Ophiacodon only, so far as has
been observed, does it enter the supraglenoid fossa back of the bor-
der; more usually, as in many modern reptiles, it is on the outer face
of the scapula in front of the border, at a variable distance above
the glenoid surface. A small artery traverses it, according to Dou-
thitt.

In the early cotylosaurs and theromorphs (Fig. 106) the glenoid
articulation is more or less spiral or "screw-shaped." In most other
reptiles it is a simple, oval cavity. In the pterosaurs (Fig. 109) it is
saddle-shaped, concave in the dorsoventral, convex in the conjugate,
diameter, permitting motion of the arm in two planes only, dorso-
ventral and antero-posterior.

The double coracoids are never elongated transversely. Turned
inward at nearly a right angle from the plane of the scapula, they
were approximated along their mesial borders (Fig. 96 d) , as shown
by many specimens in which they have been found in place. Doubt-
less epicoracoid cartilages occupied the interval in front.

In the single coracoid of later reptiles the glenoid articulation has
been completed from behind. In the modern lizards there are emar-
ginations of the mesial border (Fig. 99) , the deeper one opposite the
supracoracoid foramen; this emargination is very variable in the
mosasaurs. It has also been observed in the procoracoid of the
theromorphs. The coracoid of the Pterosauria (Fig. 109 a), Chel-
onia (Fig. 109 b), and Crocodilia (Fig. 112) is elongate. When the
sternum is present the coracoid articulates with its anterior lateral
border.

The coracoids, presumably the precoracoids only, are extraordi-
narily developed in the Plesiosauria (Fig. 102), where they sheathe



138



THE OSTEOLOGY OF THE REPTILES



the whole under side of the pectoral region, meeting in a firm median
symphysis; in most plesiosaurs throughout their lengths, but in the




Fig. 109. Pectoral girdles: A, Nyctosaurus (Pterosauria). One
half natural size. B, C, Testudo (Chelonia). One half natural
size. D, Stegochelys (Chelonia). After Jaekel. One fourth
natural size.

Elasmosauridae broadly separated posteriorly by a deep emargina-
tion, apparently a specialization (Fig. no).

In most reptiles the single coracoid is fused with the scapula in



THE PECTORAL AND PELVIC GIRDLES



139



adult life, but it is free in the crocodiles, and more or less suturally
loose in the early pterosaurs, dinosaurs (Fig. 113), phytosaurs (Fig.
in), and rhynchocephalians.




The scapula of the plesiosaurs (Figs. 102, no) is peculiar in the
development of a strong proscapular process projecting downward^
forward, and inward, and often meeting its mate in a median
symphysis, a character, unique among vertebrates. The blade is



I40 THE OSTEOLOGY OF THE REPTILES

short and small, of little service for muscular attachment, unlike the
scapulae of tail-propelling aquatic reptiles.

Probably the great development of the ventral elements of the
pectoral and pelvic girdles in the plesiosaurs implies greatest de-
velopment of the ventral muscles, used in the antero-posterior and
downward movement of the paddles. A clavicular process of the
coracoids of the later plesiosaurs (Fig. 102) extends forward to articu-
late with the proscapular process or with the clavicles. The mode of
development of the proscapular process, as shown by Andrews,
proves that it is an exogenous process of the scapula, corresponding
to the acromion and not to the procoracoid, as it was once thought
to do. The scapulae of tail-propelling aquatic reptiles are always
short and broad, fan-shaped (Figs. 85, 112). The scapula of the
Chelonia is also peculiar (Fig. 109 b, c, d). Enclosed within the
thoracic cavity it has two rather slender branches, one extending
toward the roof; the other, the proscapular process, springing from
near the articular fossa, is directed downward and inward to be
attached by ligaments to the interclavicle or entoplastron. Formerly
this process was also supposed to be a separate ossification, the pro-
coracoid, fused with the scapula, and on the strength of it a rela-
tionship was found with the plesiosaurs. It is now known to be an
exogenous process of the scapula. The coracoid is more or less
flattened and dilated at its extremity. It is directed inward and
backward, and is connected with its mate by ligaments. In Stego-
chelys, a Triassic turtle, the proscapular process is small (Fig. 100).

In Eunotosaurus , a Permian genus of South Africa, that has been
referred to the Chelonia in a wide sense, the pectoral girdle is of the
primitive type, having a moderately long scapula, slender clavicles,
and interclavicle, and the two coracoids approximating their mates
in the median line.

A distinctly differentiated acromion process occurs in reptiles only
among the Pariasauridae and especially the therapsids, mammal-like
forms from South Africa. A distinct angular process on the front
miargin of the scapula in the Cotylosauria (Fig. 96 b, c) and Thero-
morpha (Figs. 96 d, 106), to which the clavicle is attached, however,
corresponds to the acromion.

In general, the shorter and stouter are the legs, the shorter and
broader are the scapulae. In upright-walking reptiles the scapula is




Fig. 113

Fig. III. Scapula and coracoid of Rutiodon carolinensis, an American phytosaur. After
McGregor.

Fig. 112. Scapula {sc) and coracoid {cor) of gavial (Crocodilia),

Fig. 113. Pectoral girdles (Dinosauria) : A, Gorfoj««r«j- (Saurischia). After Lambe. One six-
teenth natural size. B, Allosaurus (Saurischia). After Gilmore. About one twelfth natural
size. C, Triceratops (Ornithischia). After Marsh. One sixteenth natural size. D, Morosaurus
(Saurischia). After Marsh. One twenty-eighth natural size.

141



142 THE OSTEOLOGY OF THE REPTILES

more elongated, in bipedal forms slender. The scapula of the Cotylo-
sauria (Figs. 95, 96, b, c) is relatively short and broad; that of the
Theromorpha (Figs. 98, 106) more elongated, but never narrow; that
of the therapsid reptiles (Fig. 107) relatively narrow, slender in the
Dromasauria. The scapula of the Sauropoda (Fig. 113 d) is rela-
tively long, that of the Predentata (Fig. 113 c) is much more slender,,
but it is most slender and bird-like of all in the Theropoda (Fig.

113 A, b). The scapula of the Pterosauria (Fig. 109) is always elon-
gated, very slender and bird-like in some of the earlier forms, but
stouter and firmly fused with the coracoid in the latest. In the most
specialized of all pterodactyls {Pteranodon, Ornithocheirus) its en-
larged distal extremity articulates with the fused spines of the dorsal
vertebrae, the only known examples among vertebrates of the articu-
lar union of the pectoral girdle with the spinal column.

In the early reptiles the scapula was more nearly erect, or with
a slight inclination backward. In the Crocodilia, Pterosauria, and
bipedal .reptiles, as also birds especially, it is very obliquely placed,,
the upper end turned backward over the ribs.

The Pelvic or Hip Girdle

(Figs, i 14-127)

The pelvic girdle or pelvis, in reptiles, as in other air-breathing ver-
tebrates, is composed of three bones on each side, more or less firmly
coossified in the adult, and collectively known as the innominate; the
girdle is completed by the sacrum on the dorsal side with which the
pelvis is never closely united in reptiles, not even in the Pterosauria.
The upper or dorsal bone of the three, that to which the sacral ribs or
transverse process of the lumbar vertebra are attached, is the ilium;
the one on the lower or ventral side in front is the pubis; that on the
ventral side behind is the ischium. On the outer side, where the
three bones meet, there is a cup-like depression, sometimes a hole,
the acetabulum, for the articulation of the thigh bone. In only two
groups of reptiles, the Crocodilia and Plesiosauria, is the pubis ex-
cluded from union with the ilium. In the snakes and snake-like
lizards there are at most only vestiges of the pelvic bones.

The pelvis of the terrestrial temnospondylous amphibians (Fig.

1 14 a) is ' almost indistinguishable from that of the contemporary



THE PECTORAL AND PELVIC GIRDLES



143



cotylosaur reptiles in early Permian times. The ilium of the rhachi-
tomous forms is not dilated above, as in the reptiles, but even this
distinction fails in the more nearly allied embolomerous Cricotus, in
which the ilium is prolonged backward, quite as in the reptiles. The
pubes and ischia meet in a close symphysis without openings of any
kind, except the pubic foramen, a small hole through the pubis below




Fig. 114. Pelvic girdles: A, Cacops (Temnospondyli), from below. One half natural size.
B, Seymouria (Cotylosaur), from below. A little more than one half natural size, C, D,
Varanops (Theromorpha), below and from the side.

the margin of the acetabulum, in front of the ischiatic suture, for the
passage of the obturator nerve. This "plate-like" structure of the pel-
vis is characteristic of the Cotylosauria (Figs. 114 b, 115), and more
or less of the Theromorpha (Figs. 114 c, 117), Therapsida (Fig. 119),
Proganosauria, the Choristodera, and early Rhynchocephalia.




Fig. 1 1 6. Limnoscelis paludis. Diagram of pel-
vis, from the side. ;V, ilium; ^;^, pubis; /V, ischium.



Fig. 115. Limnoscelis paludis (Cotylosauria). Pelvis,
from below. Two fifths natural size. Cross-section
through pubes at a\ cross-section through ischia at b.




Fig. 117. 0/>/^«afO(fo« (Theromorpha). Pelvis. One half natural
size. A, from the side; B, from above, pu, pubis; /'/, ilium; is,
\ schium.



144




THE PECTORAL AND PELVIC GIRDLES



145



A small opening soon appeared where the four bones meet below
in the Theromorpha (Figs. 114 c, 117), and increased in size, till, in




Fig. 118. Pelves and sacrum: A, Varanus (Lacertilia), from the right. ^, Erythrosuchus
(Parasuchia), from the right. After Broom. One tenth natural size. C, Rutiodon (Phyto-
sauria), from below. After McGregor. One eighth natural size. D, Nyctosaurus (Ptero-
sauria), sacrum and right innominate bone from within; D , anterior parasternal ribs of
same ; D , prepubis of the same from below.


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