ANNIVERSARY ADDRESS OF THE PRESIDENT. 45
Are there discernible in the members of these two orders such
structural resemblances as command our assent to their common
ancestry ?
Disregarding Placodas, as too imperfectly known, and limiting the
comparison to the elements of their shoulder-girdle and limbs, or, in
Prof. Owen's words, to their " sterno-coraco-scapular frames " and
limbs, there are present, in all the genera of each order in which
these parts are known, two pairs of bones the essential nature and
identity of which are undoubted, the coracoids and scapulae.
Fig. 1. — Shoulder-girdle of Ichthyosaurus, ventral view.
ci, the ciavicie; ic, tiie interelavicle ; sc, tiie scapula; cor, the coracoid ;
gl, the glenoid fossa.
In Ichthyosaurus (fig. 1) the coracoids are relatively broad, and they
are deeply notched in front and behind the glenoid fossa. In the
typical Plesiosauri (as P. dolichodeirus) (fig. 2) the coracoids are long
and narrow. In Cope's subgenus Elasmosaurus, as also in P. Manseli,
Hulke, and Colyrnbosaurus, Seeley, which are evidently closely related
to it, the bony coracoid is broad, but of relatively small antero-posterior
extent, and it sends backwards from the postero-external angle a
wing-like process or cartilage leaving a vacuity perhaps subtended
by membrane.
In Nothosaurus a deep narrow notch separates a long square pro-
cess in front of and internal to the glenoid part of the bone, which
a constriction divides from a large rhomboidal expansion that meets
the other coracoid in the middle line. Yet under these diversities of
form, the true homology of the coracoid is too plain to be missed ; and
this is equally true of the scapula, to which I shall presently return.
Ichthyosaurus, our sole representative of the order Ichthyopterygia,
has in its shoulder-girdle a third pair of bones, by general consent
clavicles. Are these bones present in the Plesiosaurian shoulder-girdle
and in that of the other genera of Sauropterygia ? Some anatomists
answer this affirmatively, finding clavicles in Plesiosaurus (fig. 2) in
that piece which extends forwards and inwards from the glenoid part
of the scapula to the median azygos bone reputed episternum or inter-