KEPTILIA. 235 



fore-limbs constructed upon the type of the " wing " of birds, 

 and in no living Reptile is there the bone which is known in 

 Birds as the " tarso-metatarsus." 



The class Reptilia includes the Tortoises and Turtles ( Ghe- 

 lonia), the Snakes ( Ophidia), the Lizards (Lacertilia), and the 

 Crocodiles [Groeodilia). With the exception of the Tortoises 

 and Turtles, they are mostly of an elongated cylindrical form, 

 furnished behind with a long tail. The limbs may be alto- 

 gether absent or quite rudimentary, as in the Snakes, but in 

 almost all the higher members of the class there are two pairs 

 of limbs, which may be either adapted for walking or swim- 

 ming, and which in some extinct forms support a flying mem- 

 brane. The internal skeleton is always bony, never cartila- 

 ginous or semi-cartilaginous as in many of the fishes. The 

 skull is joined to the spine by a single articulating surface (or 

 condyle). The lower jaw is complex, each half being com- 

 posed of several pieces united by sutures. In Tortoises and 

 Turtles, however, these separate pieces are amalgamated to- 

 gether, and the two halves are also united, so that the whole 

 lower jaw appears to form a single piece. In most ReptUes, 

 on the other hand, the two halves of the lower jaw (Fig. 116) 

 are only loosely united ; in the Snakes by ligaments and mus- 

 cles, in the Lizards by gristle, and in the Crocodiles by suture. 



FiQ. 116.— SknU of a Serpent (IVthon). a Qnadrate bone ; i Lower jaw, articulating with 

 the movable qnadrate bone. 



In all, the lower jaw is jointed to the skull by means of a 

 special bone, called the " quadrate bone ; " and as this often 

 projects backward, the opening of the mouth is often very 

 extensive, and may even extend backward beyond the base of 

 the skull (Fig. 116, a). Teeth are generally present, but these 

 are used chiefly to hold the prey, and not in biting or chewing 



