File:The Quarterly journal of the Geological Society of London (13205080823).jpg

ANNIVERSARY ADDRESS OF THE PRESIDENT. 47
K. Parker has shown, the true clavicles are to be found in the
episternal elements of the plastron (dermal ossifications) ; the reputed
Fig. 3. — Shoulder-girdle of Chelone mydas, dorsal view, (Slightly
altered, after Parker.)
cl, the clavicle; ic, the inter clavicle ; per, the prascoracoid ; c, the coracoid
clavicles, which are parts of the cartilaginous endoskeleton, are then
the praccoracoids ; whence it follows that there is a degree of
probability, almost amounting to proof, that the corresponding bar in
Plesiosaunis is also praecoracoid, and thus evidence of the presence of
true clavicles in this genus and, I may add, Nothosaurus, fails ; so
that in the present state of our knowledge on this point we are
warranted in considering clavicles to be wanting in Sauropterygia.
There remains for comparison one more piece, the azygos median
bone called episternum or interclavicle.
In Ichthyosaurus (fig. 1) this T-shaped bone so closely agrees in its
form and its relations with the interclavicle of existing Lizards and
Chelonia that its homology with this is unimpeachable. Can the same
be maintained of the analogous piece in the shoulder-girdle of Plesio-
saurus ? I submit that its homology with the lacertilian interclavicle
is more than doubtful. The Lizard's interclavicle is a membrane
bone ; it lies superficially (fig. 4). In Ichthyosaurus, as in extant
Lizards, it lies at the under surface of the other (endoskeletal) elements
of the girdle. In Plesiosaunis (fig. 2) the reputed clavicle lies above