GENERAL CHARACTERS OF REPTILES. 355 



and the two rami are also anchylosed. In most Reptiles, how- 

 ever, the two rami of the lower jaw are only loosely united 

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

 by fibro-cartilage, and in the Crocodilia by a regular suture. 

 In all, the lower jaw articulates with the skull by a quadrate 

 bone (fig. 312, a) ; and as this often projects backwards, the 



b 



Fig. 312. Skull of a Serpent (Python), b Articular portion of the lower jaw ; 

 a Quadrate bone ; c Squamosal portion of the temporal bone. 



opening of the mouth is often very extensive, and may even 

 extend beyond the base of the skull. Teeth are usually pre- 

 sent, but are not sunk in separate sockets or alveoli, except in 

 the Crocodiles and in some extinct forms. In the Tortoises 

 and Turtles alone of living types there are no teeth, and the 

 jaws are simply sheathed in horn, constituting a kind of beak 

 like that of a bird. 



Ribs are always present and always well developed, but they 

 differ much in form. It is not correct, however, to regard the 

 presence of ribs as separating the true Reptiles from the Am- 

 phibia, as is sometimes stated. Some of the most Lizard-like 

 of the Amphibians, such as the Siren, possess short but well- 

 developed ribs, and rudiments of ribs are traceable in other 

 orders; whilst in the Cczrilice they are large and well developed. 



As regards the exoskeleton, all Reptiles have horny epider- 

 mic scales, and they are divided into two great sections called 

 respectively Squamata and Loricata according as the integu- 

 mentary skeleton consists simply of these scales, or there are 

 osseous plates developed in the derma as well. In the Tor- 

 toises, the epidermic plates unite with the bony exoskeleton 

 and with the true - endoskeleton to form the case or box in 

 which the body of these animals is enclosed. 



The class Reptilia is divided into the following nine orders, 

 of which the first four are represented by living forms, whilst 

 the remaining five are extinct : 



