Page:North American Plesiosaurs- Elasmosaurus, Cimoliasaurus, and Polycotylus.pdf/5

Rh they are distinguishable by more than generic characters, for the Elasmosaurus is a distinct one, though its characters are not quite those assigned to it by Professor Seeley. I may add that so far, from a prolonged study of the American specimens and descriptions, I am of the opinion that no single species of American plesiosaurs can be placed in any known European genus.

Unfortunately, the type specimen of Elasmosaurus no longer has the girdles described by Cope. What has become of them is not known. There are some parts of these, especially the clavicular arch, that are necessary for a correct understanding of the genus. However, from an attentive study of this type specimen and of several other specimens which can be with much probability, if not certainty, referred to the same species, I am enabled to give the following characters for the genus Elasmosaurus. Those characters derived from the type specimen or type species are given in italics; those derived from other species referred to the genus, in roman:—

''Elasmosaurus. of short; teeth. Neck with seventy-six true cervical vertebral and three pectorals, the centra increasing in length to the fifty-eighth, and then decreasing to the dorsals; thence nearly uniform through the thoracic region; posterior cervicals and dorsals much wider than high, and wider than long; spines of vertebræ wide and not high; weak. Pectoral girdle with large meeting each other in the middle line. No .''

'' broadly separated posteriorly to the thickening, the posterior end not much dilated. Cervical ribs single-headed. short.'' Skull short;  much elevated;  bones ;  separated by. Cervical vertebræ from sixty to seventy-six in number. Scapulæ approaching or meeting in middle line. bones short; two bones only, not wider than long;  much elongated.

 Cope. Fort Pierre Cretaceous of Kansas.

A detailed description of this specimen, in completion or correction of that given by Cope, will be given later. Certain measurements and remarks may be appropriate here.

The very broad, depressed, posterior cervical vertebræ of the posterior third, or say the posterior seven feet, prohibited much motion in the living neck, either vertically or horizontally. The of the neck practically ceased at the fifty-eighth vertebra. Thenceforward the neck was more slender, very slender toward the head. It was to this part that most, if