Page:North American Plesiosaurs- Elasmosaurus, Cimoliasaurus, and Polycotylus.pdf/14

234 all improbable that the validity of the generic name may be eventually called into question, since there seems to be no difference between the teeth of this form and that described by Leidy years before, from the Cretaceous, presumably Niobrara,, as . However, as the identity must remain for many years, if not always, more or less doubtful, it would be very unwise to make any changes at the present time.

The Yale specimen, presenting as it does not a few interesting morphological and structural characters, will be fully described and figured later. It comprises the larger part of the skeleton, with the lower jaws, parts of the skull, teeth, etc. From the study of this specimen, supplemented by other specimens, clearly conspecific, the generic characters may be stated as follows:—

''Polycotylus. Teeth rather slender, with numerous well-marked ridges. Face with slender beak. Cervical vertebræ twenty-six in. number; dorsals twenty-eight or twenty-nine, inclusive of three pectorals; all short and all of nearly uniform length. Chevrons articulating in a deep concavity; all the vertebral, and especially the cervicals, rather deeply concave, and with a broad articular rim. Pectoral girdle with distinct clavicles, interclavicles, and interclavicular foramen; the scapulae not contiguous in the middle. Coracoid with a long anterior projection, united in the middle, back of the inter glenoid bar, to the posterior margin; a foramen on each side, back of interglenoid thickening. Ischia elongated. Paddles, with four epipodial bones, all much broader than long.''

The foregoing characters, it will be seen, are very much like those already given by me for , and I am somewhat in doubt as to the validity of that genus, or rather of Trinacromerum Cragin, of which, as I suspected, Dolichorhynchops is a synonym. The only important distinctions are the deep concavity of the centra and the mode of articulation of the chevrons. In none of the known species of Trinacromerum are there more than three epipodial bones, while in the two species referred to Polycotylus there are four well-formed ones. This may be, in addition to the vertebral characters, sufficient to distinguish the genera.

I give herewith some additional figures of Polycotylus latipinnis, made from the Yale specimen 1125 (Plate III, figure 1). The pelvic girdle, as will be seen, is remarkable for the great elongation of the ischia. The paddle figured by me in my previous paper on the plesiosaurs was correctly assigned to the species, but is a hind paddle instead of a pectoral limb.