238 THE VERTEBRATE SKELETON. 



Except in very young individuals the epidermal scales are 

 rubbed off from these scutes, which consequently come to pro- 

 ject freely on the surface of the body. Each scute is a nearly 

 square bony plate, deeply pitted or sculptured, and marked by 

 a strong ridge on its dorsal surface, while its ventral surface 

 is smooth. Contiguous scutes are united to one another by 

 interlocking sutures. 



The scutes are arranged in two distinct areas, viz. (1) a 

 small anterior nuchal shield which lies just behind the head 

 and is formed of six large scutes more or less firmly united 

 together, and (2) a larger posterior dorsal shield covering the 

 whole of the back and anterior part of the tail, and formed 

 of smaller scutes, which are arranged in regular transverse 

 rows, and progressively diminish in size when followed back. 



The teeth are exoskeletal structures, partly of dermal, 

 partly of epidermal origin. They lie along the margins of 

 the jaws and are confined to the premaxillae, maxillae and 

 dentaries. They are simple conical structures, without roots 

 each is in the adult placed in a separate socket, and is replaced 

 by another which as it grows comes to occupy the pulp cavity 

 of its predecessor. In the young animal the teeth are not 

 placed in separate sockets but in a continuous groove. This 

 feature is met with also in the Ichthyosauria. The groove 

 gradually becomes converted into a series of sockets by the 

 ingrowth of transverse bars of bone. The anterior teeth are 

 sharply pointed and slightly recurved, the posterior ones are 

 more blunt. 



The upper jaw bears about nineteen pairs of teeth, the 

 lower jaw about fifteen pairs. The largest tooth in the upper 

 jaw is the tenth, and in the lower jaw the fourth. 



The three living families of Crocodilia, the Crocodiles, 

 Alligators and Garials, can be readily distinguished by the 

 characters of the first and fourth lower teeth. In Alligators 

 both first and fourth lower teeth bite into pits in the upper 

 jaw ; in Garials they both bite into notches or grooves in 



