68^ Professor O^ven oti Britigh Fossil Heptiles, 



In these eocene beds, accumulated in some localities to the 

 thickness of 300 feet and upwards, the remains of crocodiles, 

 tortoises, trionyxes, turtles, and large serpents, are more or 

 less common. These fossils severally exhibit well-marked 

 and unequivocal specitic differences when compared with the 

 bones of their known existing congeners ; but their osteology 

 does not present any modifications of generic value. The 

 nearest approach to this degree of deviation occurs in the 

 eocene chelonian reptiles, as in that species of turtle from 

 Sheppey, which combines the jaws of a trionyx with the bony 

 helmet of a turtle, and presents an extent of ossification of the 

 buckler nearly equalling that of an emys. The eocene croco- 

 dile exhibits all the characters of the osseous and dental sys- 

 tems which distinguish the genus as restricted in the latest 

 systems of erpetology ; and whilst it cannot be identified with 

 any known species, most resembles, not the commonest and 

 nearest existing crocodile, as that of the Nile, but a rarer and 

 more remote one, viz., the Crocodilus Schlegelii of Borneo. 

 Not any species of reptile of the tertiary strata has been dis- 

 covered in the chalk upon which those strata immediately rest. 



A small lizard, closely corresponding in vertebral structure 

 with existing species, but differing in its dentition ; and a gi- 

 gantic marine species {Mosasaurus)^ which is the first, in the 

 present descending survey, to offer osteological and dental 

 combinations wholly unknown in existing saurians, — consti- 

 tute the representatives of the lacertian order in the creta- 

 ceous beds, which form the most recent of the secondary de- 

 posits. 



In tracing upwards the extinct reptiles, we find that the 

 union of the vertebrae by a hinder ball received into an ante- 

 rior cup, a structure which, with an insignificant exception — 

 the gecko — ^prevails throughout the saurian order as it now 

 exists, commences with the lacertian reptiles which perished 

 during the deposition of the chalk, and, in the crocodilian and 

 ophidian reptiles, is first found in the species which made their 

 appearance during the deposition of the London clay. 



Of the crocodilian order 1 have yet seen no unequivocal re- 

 pi fsentatives from the British chalk. 



All tUe well-determined Chelonians of the cretaceous period 



