ox BRITISH FOSSIL REPTILES. 193 



A small lizard, closely corresponding in vertebral structure with existing 

 species, but differing in its dentition ; and a gigantic marine species (3fosa- 

 saiirus), which is the first, in the present descending survey, to offer osteolo- 

 gical and dental combinations wholly unknown in existing Saurians, — con- 

 stitute the representatives of the Lacertian order in the cretaceous beds, which 

 form the most recent of the secondary deposits. 



In tracing upwards the extinct Reptiles we find that the union of the ver- 

 tebras by a hinder ball received into an anterior 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 du- 

 ring the deposition of the chalk, and, in the Crocodilian and Ophidian Rep- 

 tiles, is first found in the species which made their appearance during the 

 deposition of the London clay. 



Of the Crocodilian order I have yet seen no unequivocal representatives 

 from the British chalk. 



All the well-determined Chelonians of the cretaceous period are marine 

 species, and are equally distinct, with the Lacertians, from those of the super- 

 imposed tertiary beds. 



The most interesting fact which the Palaeontology of the cretaceous period 

 has yielded, with reference to the great Saurian division of the class of Rep- 

 tiles, is the commencement, or rather the last appearance of the fossil remains 

 of an order of Reptiles {Enaliosauria) now altogether extinct. I have de- 

 termined portions of the lower jaw with teeth of a large species of Ichthyo- 

 saurus from the lower chalk between Folkstone and Dover, which is very 

 closely allied to, if not identical with, the Ichthyosaurus communis. The femur 

 of a large Plesiosaurus has been obtained from the chalk of Shakspeare's 

 Cliff. Remains of more than one species of Plesiosaurus occur in the Gault, 

 and are associated with the Ichthyosaurus in the greensand near Cambridge, 

 and in the Kentish Rag near Maidstone. 



In the greensand also we first meet with evidences of Reptiles exhibiting 

 modifications of structure, especially of the locomotive extremities, as remark- 

 able and as different from those of existing species as are presented by the 

 Enaliosauria, but pointing as strongly to an adaptation for terrestrial life as 

 does the Enaliosaurian structure to aquatic existence. The specimen of the 

 unquestionably terrestrial Saurian here alluded to, viz. the Iguanodon, is the 

 more remarkable in the subcretaceous marine strata, in consequence of its pre- 

 senting the largest proportion of the connected skeleton of the same indivi- 

 dual of this species tliat has hitherto been found. 



Gigantic Ci-ocodilian Reptiles, removed by generic modifications of struc- 

 ture from the Eocene and existing Crocodiles, now likewise begin to be in- 

 dicated, as by the teeth of the Polyptychodon from the greensand quarry at 

 Maidstone, and by the large bones of the extremities from the quarries of a 

 corresponding stratum at Hythe. 



The Chelonian from the greensand (Chelone pulchriceps) differs from the 

 Eocene and all existing turtles in a very interesting modification of the anatomy 

 of the cranium. 



In the Wealden group of freshwater strata, the Enaliosaurian order con- 

 tinues to be represented by the Plesiosaurus, but no remains of the more 

 strictly marine genus. Ichthyosaurus, have yet been discovered. This cir- 

 cumstance corresponds with the more strict adaptation for marine existence 

 which the structure of the Ichthyosaurus presents, and corroborates the in- 

 ference that the Plesiosaurus lived nearer the shore, and ascended estuaries. 

 The re-appearance of the Ichthyosaurus in the chalk formations proves that 

 it had continued to exist in the neighbouring ocean, and indicates, perhaps, 

 ISll. o 



