This genus is well distinguished from Plesiosaurus by the peculiarity of the scapular arch. The mesosternum appears to be coössified with the claviculi, and the three elements form a broad breast-plate. If the claviculus was ever united with the scapula as in Plesiosaurus, no evidence of it can be seen in the specimen. Both the clavicular and mesosternal elements are broader and more extended anteriorly.
The American genera of Elasmosauridæ may be compared as follows :
Posterior cervical vertebræ without diapophyses: cervicals longer, compressed, neck very elongate.
Posterior cervical vertebræ with diapophyses : cervicals quadrate, shorter, depressed, rapidly diminishing in size, hence the neck shorter.
Prof. Owen figures and describes (Reptiles of the Cretaceous, Palæontogr. Soc.) a vertebra which very closely resembles the cervical of Elasmosaurus. He considers it to be the cervical of a peculiar Plesiosaurus, which he calls P. constrictus, remarking, at the same time, its remarkably inferior pleurapophyses. This I believe to be a species of Elasmosaurus or an ally, and to be called for the present Elasmosaurus constrictus.
ELASMOSAURUS PLATYURUS, Cope.
Leconte's Notes loc. cit. Proceed. Acad Nat. Sci., 1868, 1. c. 92.
Discosaurus carinatus, Cope. Leconte's Notes, 1. c.
This, after Mosasaurus the most elongate of the sea saurians yet discovered, is represented by a more than usually complete skeleton in the Museum of the Academy of Natural Sciences in this city. It was found by Dr. Theophilus H. Turner, the physician of the garrison at Fort Wallace, a point situated 300 miles westward from Leavenworth on the Missouri river, and some distance north from the Smoky Hill Fork of the Platte river. Portions of two vertebræ presented by him to Dr. Leconte when on his geological tour in the interest of the U. S. Pacific Railroad Company, were brought by the latter gentleman to the Academy, and indicated to the writer the existence of an unknown Plesiosauroid reptile. Subsequent correspondence with Dr. Turner resulted in his employing a number of men, who engaged in excavations, and succeeded in obtaining a large part of the monster. Its vertebræ were found to be almost continuous, except a vacancy of some four feet in the interior dorsal region. They formed a curved line, a considerable part of whose convexity was visible on the side of a bluff of clay shale rock, with seams and crystals of gypsum. The bones were all coated with a thin layer of gypsum, and in some places their dense layer had been destroyed by conversion into sulphate of lime.
The scapular arch was found in large part adhering to the bodies and neural spines of a series of the anterior dorsal vertebræ, and was detached from it at the Academy. The pelvic arch had been slightly crushed, and the lumbo-sacral vertebra forced into contact with the ischia, where they remain. A broken extremity of the supposed ilium was forced into the matrix which supports the ischia. Many of the dorsal and caudal vertebræ were sent, and remain in continuous masses, so that the succession is readily traced, and the trace relations of the extremities preserved.
In removing the matrix from beneath. the vertebræ, scales and teeth of sonme six species of Physoclyst and Physostomous fishes were found, including an Enchodus and a Sphyraena, the latter indicating a new species, which I have called S. carinata. These animals had doubtless been the food of the Elasmosaurus.
