These powerful extremital pieces indicate a body to be propelled, of not less than usual proportions. If this be the case the number of dorsal vertebræ is considerably greater than in the species of this order in general, and approaching more the Ichthyosauri. I do not intend to suggest any affinity between the latter and the present genus, as none exists. What the extent of cervical vertebræ may have been is uncertain. The caudals have probably been numerous, though not probably so extended as in Elasmosaurus.
The size of the species can be approximately estimated from the proportions furnished by Owen (Reptiles of the Liassic Formations) for Plesiosaurus rostratus. The skeleton of this species measures 11 feet 8 inches, and the dorsal vertebræ are of less vertical and equal transverse diameter compared with those of the present Saurian. We may therefore suppose that the latter exceeded the former in dimensions. William E. Webb of Topeka discovered the specimens on which this species rests, and liberally forwarded them to me for examination and description.
ISCHYROSAURUS, Cope.
Ischyrotherium Leidy. Trans. Amer. Philos. Soc , 1860, 150.
This genus has been referred by Leidy to the Mammalia, and to the order Sirenia, with doubt. Having access to a part of the remains on which it was established, I have arrived at the conviction that it really represents an aquatic Saurian more or less distantly related to Plesiosaurus. My reasons for regarding it as Reptilian and not Mammalian are : first, the articulation of this neural arch with the centrum ; second, the absence of epiphyses ; third, the absence of articulation for the head of the rib on the centrum ; fourth, the lack of tuberculum on the ribs.
With respect to the first of these characters, it may be remarked that it never exists in mature Mammalia, and disappears at an early period of the development of all, except in certain seals and the Echidna, where the consolidation of the neural arch is a little delayed. As to the epiphyses, there is no trace of their suture to be found on fractured surfaces, supposing their existence to be indicated by the series of foramina extending on the inferior surface of the centrum near each articular extremity. These foramina are, I believe, merely the ruptured coarse cells, which can be found near the articular faces in the vertebræ in all Sauropterygia. They are unusually small in this genus, appropriately to the denser structure of the bones as compared with other sea saurians. The articulation of the rib takes place at the extremity of a long diapophysis, and there only, there being no pit for the capitulum. This does not occur in Mammalia, but is highly characteristic of the lower groups of the Reptilia, especially the Sauropterygia. The lumbar series in Cetacea presents a somewhat similar structure. The vertebræ in question are referred by Leidy to this position, but they are clearly median dorsals, from the elevated position and length of the diapophyses. The simple form of the ribs, some of which are from the same part of the column, is quite unknown among Mammalia.
There are other significant characters of less value, which point to the saurian affinities of this genus, and confirm the preceding. These are the very small size of the neural canal, the cylindric or thickened form of the neural arch, and the strong venous foramina
