23-t 



PALAEONTOLOGY 



large extinct nataix>ry group of saurian reptiles, the essential 

 distinctness of the groups Sa/wropterygii and IcMh/yopterygii, 

 typified by the Ichthyosaurus and Plesiosaurus respectively, 

 should be borne in mind. 



Sp. Pliosaurus brachydeirus, Ow. — The generic characters 

 of Pliosaurus are given by the teeth and the cervical vertebrae. 

 As compared with those of Plesiosaurus, the teeth are thicker 

 in proportion to their length, are subtrihedral in transverse 

 section, with one side flattened, and bounded by lateral promi- 



Fig. 72. 



Pliosaurus (Kimmeridgian). 



nent ridges from the more convex sides, which are rounded off 

 into each other, and alone show the longitudinal ridges of the 

 enamel ; these are there very well defined. The vertebrae of 

 the neck, presenting a flat articular surface of the shape six iwn 

 in outline below the neck in fig. 72, are so compressed from 

 before backward as to resemble the vertebras of the Ichthyo- 

 saurus (fig. 70, c), and as many as twelve may be compressed 

 within the short neck intervening between the skull and 

 scapular arch, as shown in fig. 72. For the rest, save in the 

 more massive proportions of the jaws and paddle-bones, the 

 bony framework of Pliosaurus closely accords with that of 

 Plesiosaurus ; and, as the vertebra? of the trunk resume the 

 plesiosaurian proportions, they give little indication of the 

 genus of reptile to which they truly belong, when found detached 

 and apart Some individuals of Pliosaurvs appear to have 

 attained a length of between 30 and tO feet. The remains of 



