304 PALAEONTOLOGY. 



Teleosauri in being somewhat thicker in proportion to their 

 length, and larger in proportion to the jaws. 



The fossil jaws of the extinct Crococlilians demonstrate 

 that the same law regulated the succession of the teeth at the 

 ancient epochs when those highly-organised reptiles prevailed 

 in greatest numbers, and under the most varied generic and 

 specific modifications, as at the present period, when they are 

 reduced to a single family composed of so few and slightly 

 varied species as to have constituted in the system of Linnaeus 

 a small fraction of the genus Laceria. 



The large, thick, externally ridged or pitted scutes, though 

 common to the Crocodilian order, are not peculiar to them. 

 The labyrinthoclont Rhombopholis* and the thecodont Stagono- 

 lepis, have left similar petrified scutes. 



Sub- Order 3. — Proccelia. 



All existing Crocodilians are procoelian. The best and 

 most readily recognizable characters by which they are 

 grouped in appropriate genera are derived from modifications 

 of the dental system. 



In the caimans (genus Alligator) the teeth vary in number 

 from J|^g to m| ; the fourth tooth of the lower jaw or canine, 

 is received into a cavity of the palatal surface of the upper jaw, 

 where it is concealed when the mouth is shut ; in old indi- 

 viduals the upper jaw is perforated by these large inferior 

 canines, and the fossae are converted into foramina. 



In the true crocodiles (genus Crocodilus) the first tooth in 

 the lower jaw perforates the palatal process of the intermaxil- 

 lary bone when the mouth is closed ; the fourth tooth in the 

 lower jaw is received into a notch excavated in the side of the 

 alveolar border of the upper jaw, and is visible externally 

 when the mouth is closed. 



* " Labyrinthodon scutulatus" Trans. Geol. Soc, 2d series, vol. vi., p. 538, 

 pi. 46. 



