10 0. C. Marsh on the Remains of a new Enaliosaurian. 



of the Ichthyosaurus. This is especially noticeable in their flat- 

 tened and subhexagonal form, in their deep and regular terminal 

 concavities, and in the separate state of the neural arch. The 

 differences which exist, however, although of much less import- 

 ance, must not be disregarded. The most marked of these have 

 already been alluded to ; and may be seen in the absence from 

 the sides of the centrum of costal articular surfaces, in the deeper 

 concavities at the vertebral extremities, and in the form and di- 

 mensions of the superior arch. The first of these differences 

 would alone be deemed sufficient, by the highest authority, to 

 establish a distinction between these remains and the vertebrae 

 of ih.Q Ichthyosaurus ; for in that genus, according to the state- 

 ment of Prof Owen, which is pecaliarly applicable to the pres- 

 ent case,—" The lower tubercle for the attachment of the rib 

 never wholly quits the centrum: any detached vertebral cen- 

 trum, therefore, that might be discovered, which had no lateral 

 tubercle or articular surface for a rib, might be safely pro- 

 nounced, whatever the form of its anterior and posterior articu- 

 lar surfaces, not to have belonged to a true Ichthyosaurus, pro- 

 vided It was not compressed laterally, as in the small terminal 

 nbless caudal vertebrse which supported the caudal fin in the 

 Ichthyosaurus:'^ The absence of any lateral compression in the 

 present remains, together with their size and proportions, prove 

 conclusively that they cannot be brought under the exception, 

 which Prof Owen makes of the terminal caudal vertebrse of the 

 Ichthyosaurus; and hence the application of his rule would separ 

 rate them from that genus. 



The points of similarity, then, between these vertebrae of the 

 Eosaurus, and those of the Ichthyosauri, which they most resem- 

 ble, clearly indicate that they belong to the same natural group 

 of marine reptiles, and to the same order; while the differences 

 which exist between them seem to be sufficiently numerous and 

 important to authorize the conclusion that they are generically 

 distinct; as might naturally be expected from the vast periods 

 of time that separated their existence. 



Since the genera of Enaliosaurians from the Secondary forma- 

 tions, although contemporaneous, differed so widely in form and 

 structure, analogy would lead us to infer that a Pateozoic rep- 

 resentative of the family would present still more marked pecu- 

 liarities in these respects. It is, therefore, particularly interest- 

 ing to find indications of so strong a resemblance between this 

 primitive saurian and the more recent IchtJiyosaurus. These 

 fossils, however, present some features of a lower and more 

 ichthyic type of structure than that genus possessed, and it is 

 not unhkely that other parts of the skeleton would show a wider 

 divergence. 



* Report on British Foadl Reptiles, Part I, page 102. 



