EEPTILIA. 207 



land. The best-kuown genus is Mosasaurus, of whicli the most 

 celebrated species is the M. Camperi (fig. 556) of the Maes- 

 tricht Chalk. Other genera belonging to this group are 

 Leiodon, Tylosaurus, Lestosaurus, Olidastes, &c. The entire 

 group, so far as at present known, is confined to the Creta- 

 ceous rocks, and though represented in Europe, it seems to 

 have had its maximum development in the North American 

 area. 



In the Tertiary rocks the remains of Lacertilians are not 

 by any means unknown, but none of the forms of this period 

 are sufficiently important to demand especial attention. Most 

 of the Tertiary Lacertilians, however, are of small size, and 

 appear to have been terrestrial in their habits, thus approxi- 

 mating to the typical existing Lizards. The most remark- 

 able group of the Tertiary Lizards, however, is that of the 

 Glyptosauridce {Glyptosaurus, and Oixosaurus), comprising 

 forms which are found in the Eocene Tertiary of North 

 America, and have the anomalous character that the skin 

 was furnished with ornamented osseous scutes. It is also 

 worthy of notice that while the Tertiary Lizards are refer- 

 able for the most part to actually existing groups, the recent 

 genus Chamcdco seems to have been in existence as early as 

 the Eocene. 



Oeder IV. Crocodilia. — The last and highest order of 

 the Kving Eeptilia is that of the Crocodilia, including the 

 living Crocodiles, Alligators, and Gavials, and characterised 

 by the following peculiarities : — 



The body is covered tvith an outer epidermic exoskeleton com - 

 posed of liorny scales, and an inner dermal cxoskeleton consisting 

 of transverse roivs of squared bony plcdes or scutes, wliicli may he 

 confined to the dorsal surface alone, or may exist on the ventral 

 surface as vjcll, and lohieh are disposed on the hack of the neclc 

 into groups of different form and numher in certain species. The 

 hones of the skull and face are firmly united together, and the two 

 halves or rami of the lower jaw are united in front hy « suture. 

 There is a single roiu of teeth, wlvich are implanted in distinct 

 sockets, and are hollowed at the base for the germs of the 

 new teeth, by which they are successively pushed out and re- 

 placed during the life of the animal. The centra of the dorsal 



