38 THE EXTINCT BATRACHIA, REPTILIA 



These powerful extremital pieces indicate a body to be propelled, of not less than usual proportions. If this he 

 the case the number of dorsal vertebrae 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 vertebrae may have been is uncertain. The eaudals 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 vertebras 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 , 1800, 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 the 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 tubercuhun 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 vertebrae in all Sauropterygia. They are unusually small in this genus, appropri- 

 ately 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 lum- 

 bar series in Cetacea presents a somewhat similar structure. The vertebrae 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 affini- 

 ties 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 



