File:The Quarterly journal of the Geological Society of London (13205249514).jpg

46 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
with the scapula. This interpretation seems to be supported by an
analogous bar in the amphibian shoulder-girdle. This, considered
Fig. 2. — Shoulder-girdle of Plesiosaurus (type), ventral view.
osty the omosternum ; pc, the prsecoracoid ; sc, the scapula ; co, the coracoid ;
H, the humerus.
praecoracoid by W. K. Parker, regarded as clavicle by Wiedersheimer,
thought to comprise both praecoracoid and clavicle by Hoffmann, is
primitively a simple cartilaginous rod extending ventrally from the
scapula towards the middle line of the body. Later, an ectosteal splint
forms on its front border, and, growing, more or less sheathes it, the rod
itself meanwhile undergoing a variable amount of endosteal ossifi-
cation ; and when these two processes of ossification are completed, a
slight longitudinal furrow may be the only trace of the dual origin
of the bar. It is evident that the ectosteal splint alone can have any
pretention to homology with the clavicles of higher vertebrates ; and
I imagine that its intimate association with the cartilaginous rod
(which is segmented off by longitudinal fissure from the coracoid and
is therefore unmistakably praecoracoid) was one of the considerations
that led Prof. Parker to regard the bar in its entirety as praecora-
coid, and to deny the occurrence of true clavicles in Batrachia, a view
which has much in favour of it. In Chelonia (fig. 3), however, the
ambiguity, so puzzling in Batrachia, disappears. They also have a
similarly placed ventral bar, which, except in very immature indi-
viduals, is (as in Plesiosaurus) confluent with the glenoid part of the
scapula, from which it extends inwards towards the middle line of
the body. Early comparative anatomists regarded this as an acro-
mion or clavicle, the view, I think, still held by Prof. R. Owen. Of
its homology with the analogous bar in Plesiosaurus and Nothosaurus
I myself have no doubt ; and if it be clavicle in the Chelonian it is so
in Plesiosaurus and Nothosaurus. and thus clavicles would be present