EXTINCT FOEMS. 139 



but little in size in different parts of tlie jaw, while 

 neither of them have an armour on the lower surface of 

 the body. They differ from crocodiles and alligators in 

 feeding chiefly on fish. 



In regard to their geological distribution, crocodilians 

 more or less closely allied to the existing garials, crocodiles, 

 and alligators, are found throughout the rocks of the 

 Tertiary period as far down as the London clay. Some of 

 these extinct species were, however, of gigantic dimensions; 

 one from the Siwalik Hills of India, which was allied to 

 the garial, attaining a length of between fifty and sixty 

 feet, and thus presenting a great contrast to living croco- 

 diles, which rarely exceed a length of some twenty-two or 

 twenty-three feet. Another species found in the Tertiary 

 clays of Hampshire presents characters intermediate 

 between crocodiles and alligators, the fourth lower tooth 

 being usually received into a pit in the skull, but the 

 under surface of the body having a complete bony armour 

 like that of the caimans. This genus (Diplocynodon) 

 differs from both crocodiles and alligators in that both 

 the third and fourth lower teeth are larger than the 

 adjacent ones, so that the animal had two powerful tusks 

 in the sides of the lower jaw. 



A few crocodilians, more or less closely allied to existing 

 types, also occur in the Cretaceous rocks, but when we 

 reach the Wealden and Jurassic strata nearly all the forms 

 differ very markedly from modern ones, and show a lower 

 stage of development. Before, however, we are in a 

 position to understand how these early crocodilians differ 

 from their hving cousins, we must enter a little more fully 

 into the anatomy of the latter. Turning once more to 

 Fig. 45, we see that the passage leading to the internal 

 nostrils is formed by four pairs of bones, on the fourth of 

 which the letter N is placed. Again, in all living croco- 

 dilians the vertebrae articulate with one another by a liall- 

 and-sock'et joint, of which the cup is situated at the front 

 of each vertebra ; this mode of articulation being the 

 best adapted to give free motion of one vertebra upon the 

 other. The third point we have to notice relates to the 

 bonv armour of living crocodilians, in all of which the 



