258 



VEETEBEATE ANIMALS. 



The class Reptilia includes, among living forms, the Tortoises and 

 Turtles (Ckelonia), the Snakes (O/Jiidia), the Lizards (Lacertilia), 

 and the Crocodiles {Crocodilia). With the exception of the Tor- 

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

 furnished behind M'ith a long tail. The limbs may be altogether 

 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 swimming, and which in some 

 extinct forms support a flying membi'ane. The internal skeleton is 

 always bony, never cartilaginous or semi-cartilaginous as in many of 

 the fishes. The skull is jointed to the spine by a single articulating 

 surface (or cnndijh-). The lower jaw is complex, each half being 

 composed of several pieces united by sutures. In Tortoises and 

 Turtles, however, these separate pieces are amalgamated together, 

 and the two halves are also united, so that the whole lower jaw 

 appeal's to form a single piece. In most reptiles, on the other hand, 

 the two halves of the lower jaw (fig. 185), are only loosely united ; 

 in the Snakes by ligaments and muscles, in the Lizards by gristle, 

 and in the Crocodiles by suture. In all, the lower jaw is jointed 



. 1S5. — Skull of a Serpent (Python), a Quadrate bone: & Lower jaw, 

 articulating witli tlie uiovalile quadialr bone (o). 



to the skull by means of .-i special bone, called the " quadrate 

 bone"; and as this often projects backwards, the opening of the 

 mouth is often very extensive, and may even extend backwards 

 beyond the base of the skull (fig. 185, a). Teeth are generally pres- 

 ent, but these are used chiefly to hold the ]irey, and not in biting or 

 chewing the food. Except in the Crocodiles, the teeth are not sunk 

 into distinct sockets, and they are usually replaced as fast as shed. 

 The_^' likewise do not differ from one another sufficiently in form or 

 function as to allow of their being divided into different sets, as they 

 can be in the Mammals. Usually the teeth are confined to the jaws 



