282 PLEIOSAURUS. 



Notwithstanding the approximation to the crocodilian type which 

 the teeth of the Plesiosaur make in their persistent pulp-cavity, there 

 is not more than a single successional tooth in progress of develop- 

 ment at the base of the tooth in use at any period ; and the dentition 

 of the Plesiosaur further differs from that of the crocodile, inasmuch 

 as the new tooth, instead of emerging from the pulp-cavity of the old 

 tooth, or even from the same socket, protrudes its apex through a 

 distinct foramen at the inner side of the alveolus of its predecessor. 



PLEIOSAURUS. 



122. Large, simple, conical teeth, with the enamelled crown tra- 

 versed by well-defined and abruptly terminated longitudinal or 

 oblique ridges, as in the Plesiosaur, have not unfrequently been dis- 

 covered in the Kimmeridge clay formation. These teeth differ from 

 those of the Plesiosaur in their greater relative thickness as compared 

 with their length and in the subtriedral shape of their crown ; the 

 outer side is slightly convex, sometimes nearly flat ; it is separated 

 from the two other facets by two sharp ridges ; these are 

 more convex, and the angle dividing them is often so rounded off, 

 that they form a demi-cone, and the shape of the tooth thus approxi- 

 mates very closely to that of the Mosasaur, with which it is equal in 

 size. It may readily be distinguished, however, even when the crown 

 only is preserved, by the ridges which traverse the inner or convex 

 sides ; the outer flattened surface alone being smooth : but an entire 

 tooth of the present extinct reptile presents a long fang, which at 

 once removes it from the acrodont group, and allies it with the 

 thecodont reptiles, among which it approaches nearest, in the super- 

 ficial markings of the crown, to the Plesiosaurus. The known parts 

 of the skeleton of the gigantic extinct reptile, to which the teeth in 

 question belong, confirm this approximation ; but the vertebrae of the 

 neck are so modified that the peculiarly elongated proportion of this 

 part of the spine, which characterizes the typical Plesiosauri, is 

 exchanged for one that much more nearly approaches the opposite 

 condition of the cervical region in the Ichthyosauri ; this abrogation 

 of the main characteristic of the Plesiosauri, combined with the 

 more crocodilian proportions of the teeth in the present reptile, have 



