Page:Synopsis of the Exinct Batrachia and Reptilia of North America. Part 1..pdf/49

Rh pressed, the centra much longer than deep, and deeper than wide, and with smooth concave sides.

The ribs of the anterior cervico-dorsal region are inserted directly in the vertically oval pits of the centrum. Immediately at the point where these cease, thin transverse processes appear to arise from the lower edges of the rib pits. They form a continuous series with the ribs, and soon rise from the plane of the lower face of the centrum, and are directed obliquely downwards. At the end of the cervical series they are directed nearly vertically downwards. The number of these vertebræ is very great, the anterior diminishing to a very small size, the whole measuring a little more than half the total length.

Most of the cervieals possess two venous foramina below ; the dorsals two, and most of the caudals one.

The resemblance of the caudals to the usual type of Plesiosaurus, is seen in the fact that each bears near its posterior articular aspect, on the inferior face, a pair of articular surfaces, for chevron bones. Similar vertebræ had been described by Leidy as the caudals of a genus he called Discosaurus, the study of the present genus shows that they are really of the caudals of the allied genus Cimoliasaurus.

The ribs are simple headed ; the abdominal ribs seen in Plesiosaurus are possibly wanting, as none were found by the discoverer of the fossil, after a careful search.

The end of the muzzle, with symphysis mandibuli, was preserved. This is flat, the symphysis rather short, the premaxillary grooved at the intervals between the dental alveoli. The teeth are deeply implanted, with small pulp cavity, are cylindric and furnished with nearly straight elongate conic crowns, which are minutely but sharply striate to the tip ; the ridges, straight, continuous. There are no indications of nostrils, so that these were probably posterior and near the orbits, as in Plesiosaurus.

The pelvic arch is more extended than the scapular, and strongly resembles the pelvic arch of other Plesiosauridæ. The scapular arch is peculiar ; the clavicles are broad flat bones resembling the pubes of certain tortoises, while the coracoids are much like the coracoids of Plesiosaurus.

The scapular arch is remarkable for the resemblance of coracoids to those of Plesiosaurus. The clavicles have a greater transverse extent than the former, and have a very extensive line of union medially, and a narrow posterior prolongation which meets a similar anterior one of the coracoids, separating the intervening foramina. They appear to form about-one third of the walls of the glenoid cavity, and have a constricted base as in some Plesiosauria, applied to the extremity of the coraeoid. The form of the glenoid cavity cannot be readily ascertained from the absence of the scapula. What we have of

AMER'. PHILOSO. SOC.-VOL. XIV. 12