File:The Quarterly journal of the Geological Society of London (12734064723).jpg

OF THE PLESIOSAUEIAN PECTOKAX AECH. 437
the precoracoids and coracoids, and a thin film of ossified connective
tissue extend over the interspace between them and unite these
bones respectively on each side of the animal, a pair of coracoid
bones will be constituted similar in form, position, and relation to
those which characterize the Plesiosauria, the only difference being
that in Plesiosaurs the precoracoid is connate with the coracoid, as
in the Ostrich and many Lizards

while in Chelonians it is connate
with the scapula (s)

but much importance cannot be attached to the
condition of the precoracoid, since no one will contend that Plesio-
saurs are either Lizards or Tortoises

while among Amphibians the
precoracoid is a distinct bone, so placed that it might combine indif-
ferently with coracoid or scapula.
It is a noteworthy fact, familiar to all who collect Plesiosaurian
coracoids from soft strata, that the whole of this triangular inter-
space between the precoracoid and coracoid parts, which I covered
in the Turtle with an imaginary thin ossification, is liable to be
broken away in extricating the fossil

and then there remains only
a curious bone forked from the humeral articular surface, which
closely resembles the precoraco-coracoid portion of the chelonian
pectoral arch just described, and in which only the precoracoid parts
similarly meet each other in the median line. A beautiful example
of such a specimen from the Kimmeridge Clay, presented by J. C.
Mansell-Pleydell, Esq., is exhibited in the British Museum (fig. 2)*.
Fig. 2. — Coracoids of Hurgenosanrus.
Showing connation of coracoid (c) and precoracoid (pc).
Such a specimen goes far to justify an interpretation of the Plesio-
saurian coracoids as the combined coracoid and precoracoid bones.
The Chelonian, however, has the form of its pectoral and pelvic
arches modified by the enormous amount of potential epiphysial
ossification which characterizes the subclass. The epipleural carti-
lages of the Crocodile and the small epipleural bones of Hatteria
and of Birds are, among Chelonians, changed by potential growth
into ossifications, in comparison with which the original pleural ele-
ments are small

while they so grow and unite that an epipleural
skeleton is formed, encased not in muscles as in most other ani-
mals, but in representatives of muscles, now converted, in most
Chelonians, into horny scutes corresponding with muscles in their
extension.

It is at present standing over the remains of Megalosaurus, without osteo-
logical determination, and is placed upside