has not attained such huge dimensions as some of the latter. These fragments do not throw much light on the structure of the pelvic arch.
The structure of the bones is, like that in the order generally, of the coarsest description. There are no medullary cavities, but the medullary cells are large, and extended everywhere in the direction of the axis of each bone. `
The characters which separate this genus from Plesiosaurus may be derived from the preceding as follows:
First; the deeply biconcave, and very short vertebral centra.
Second , the tibia broader than long, resembling those of Ichthyosaurus.
Third; the coalescence and depression of some of the cervicals.
Fourth ; the continuity of the neural arches.
Fifth ; the continuity of the diapophyses of the caudals.
The only genus with which this genus compares nearly, is the Thaumatosaurus of Meyer. This is known but by a few fragments, and of these, but few are present in the Kansas animal. The character on which I rely at present to distinguish them, is the much less concavity of the dorsal vertebræ in Thaumatosaurus. This is however, not entirely satisfactory. Thaumatosaurus oolithicus Meyer is from the lower oolite of South Germany.
The bones are thoroughly mineralized, and the adherent matrix is a light yellow chalky limestone, similar to that which yielded the fine fragments of the Macrosaurus proriger. This, Dr. Leconte informs me, is probably Meek and Hayden's upper Cretaceous No. 3, and is a higher horizon, than that near Fort Wallace from which Dr. Turner procured the Elasmosaurus platyurus. The specimens were all taken out under the direction of W. E. Webb, of Topeka, from the same spot ; from every point of view there is reason to believe that they belong to the same animal.
POLYCOTYLUS LATIPINNIS, Cope.
The anterior dorsal vertebræ have the centra slightly compressed or vertically oval, while the posterior are more rounded. The anterior caudals appear to have been round or nearly so; they are somewhat distorted by pressure. The sides of the centrum are slightly concave in the longitudinal direction; below, there is no carina, but at least two venous foramina. There is another large foramen on the side of the centrum, usually not far from the neural arch ; there are usually other smaller foramina below this. The bases of the diapophyses are longitudinally grooved behind, and separate a concavity of the arch in front of them from one behind. In the most median, the most elevated diapophysis stands about equally on the neurapophysis and the neural spine above it. The diapophysis are vertically compressed, and the costal articulation of the only one preserved, is in the same plane. The margins of the external surfaces are not coarsely striate as in many Sauropterygia. The venous foramina of the distal coössified cervicals are in pairs, and of a large size. In the proximal caudals the diapophyses are above the middle of the sides of the centra. In one the basis of a chevron is preserved. It is cylindric and striate. The zygapophysis
