Glass 4. Beptilia. Order 4. Grocodilia. 



427 



For the rest, these tkr-ee groups are connected by intermediate foi-ms ; there are 

 species of Crocodiles which approach the Alligators, and a Gavial which forms a 

 connecting link with the Crocodiles. 



The oldest Crocodiles known, from the Triassic {Belodon and others) are 

 specially characterised by their resemblance to Lacertiha and Ohelonia with 



Fig. 351. 

 pterygoid. 



A Skull of a Gavial, Bof a Teleosaiirus. n' internal nares, p palatine, v 



respect to the position of the internal nares; the palatines and pterygoids 

 do not form a canal, but the internal nares open much farther f oi-ward than in 

 those now living. The Crocodiles from the Jurassic, and some of those from the 

 Cretaceous formations [Teleosaiirus [Kg. 351 5], etc.) approach the extant 

 fonns in this respect ; for the palatines, but not the pterygoids, are connected 

 to form a canal, and the nares are thus moved much further back. All these- 

 older forms difEer also from those of to-day in the possession of biconcave 

 vertebrae. On the other hand the Tertiary and some of the Cretaceous forms- 

 exactly resemble those now existing ; the pterygoids take pai-t in the formation 

 of the nasal canal, and the centra are procoelous. The Crocodiles of different 

 periods afford a very interesting series. 



Whilst many extinct Reptiles, e.g., the Crocodiles just mentioned,, 

 as well as many others, are allied to the orders of the present day, there are 

 also many forms constituting orders which are without living representatives. 

 Several of these groups are of considerable interest, and the most important 

 will now be briefly considered. 



The Ichthyosaurians occupy a position among the Reptiles similar to that 

 of the Whales among Mammals, and they are very suggestive of this group. The 



Fig. 352. An Ichthyosanrian. 



head, especially the snout, is of huge size, the neck uncommonly short, the 

 tail very long and powerfvil ; both pairs of limbs are formed like the fins of a 

 Whale : they are short, broad plates, all the bones of which ai-e immovably 

 connected and much shortened ; the digits, of which there were often more than 

 five on each foot, were enclosed in a common skin, and without claws, and the 

 niimber of joints to each digit was very great, though each joint was veiy 



