Page:North American Plesiosaurs- Elasmosaurus, Cimoliasaurus, and Polycotylus.pdf/16

236 the propodials, the great transverse of the epipodials, and the markedly greater concavity of the posterior border of the propodials (Plate III, figure 2). The metapodials and phalanges are also notable for their shortness and robustness. There are four facets on the distal extremity of the propodials for articulation with the epipodials.

An additional species of this genus is known to me, and will be figured and described in a later communication.

The genus Trinacromerum, if it be distinct from Polycotylus, as I believe that it is, is represented in the Yale collection by a considerable portion of a skeleton (No. 1129), clearly identifiable with  Williston, from the Benton Cretaceous. This specimen, which offers some additional facts of interest, will be figured and described in a later communication, in connection with the description of the type species of the genus, , which I have recently studied in the collection.

Pelvic girdle of Elasmosaurus ischiadicus Williston. p, pubis ; il, ilium; is, ischium. Specimen No. 1130, Yale Museum.

—Eight pelvic paddle of Elasmosaurus ischiadicus Williston. Specimen No. 1130, Yale Museum.

—Left pectoral paddle of Elasmosaurus (?) marshii Williston. Specimen No. 1645, Yale Museum.

—Left half of pelvic girdle of Polycotylus latipinnis Cope, p, pubis; is, ischium ; il, ilium. Specimen No. 1125, Yale Museum.

—Right propodial and epipodial bones of Polycotylus dolichopus Williston. Specimen No. 1642. Yale Museum.

—Right humerus of Elasmosaurus snowii Williston. Specimen No. 1644, Yale Museum.

Elasmosaurus nobilis Williston. Specimen No. 1640, Yale Museum.

—Right ilium.

—Left ilium.

—Right femur.

—Posterior sacral vertebra.

—First sacral vertebra.

—Anterior dorsal vertebra.

—Posterior dorsal vertebræ.

—Middle sacral vertebra.