ORDER SAUROPTERYGIA. 



1075 



physis ; whereas among the Crocodilia the tendency has been pre- 

 cisely in the opposite direction. 



This family has been divided into a large number of genera, but 

 since several of these are not really distinct it will suffice to adopt a 

 smaller number of such divisions. The genus Plesiosaums, as now 

 restricted, is exclusively confined to the Upper Trias (Rhsetic) and 

 Lias. Owing to the beautiful preservation of many of the species 



Fig. 996. — The mandible in different genera of Plesiosauridce. Reduced, a, Peloneustes 

 fhilarchus, from the Oxford Clay, one-eighth natural size : b, Thaumatosaurus indicus, from 

 the Upper Jurassic of India, one-seventh natural size ; c, Plesiosatints dolichodirns, two-fifths 

 natural size. 



this genus has been long known to science ; and its remains were 

 admirably described in the first third of the present century by the 

 late Mr Conybeare and Dean Buckland, who with remarkable fore- 

 sight hinted at the affinity of these strange and weird forms of Rep- 

 tilian life to the Chelonia. In this genus the skull is either small 

 with a short mandibular symphysis (as in fig. 996, c), or moderately 

 large with a longer symphysis and rostrum. The teeth are generally 

 slender, without carinse, and the terminal ones are not much larger 



