CHARACTERS OF REPTILIA. 395 



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 



Fig. 153. Skull of a Serpent (Python), b Articular portion of the lower jaw ; 

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



the Crocodiles. In the Tortoises and Turtles alone 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 C&cilice. they are large and well de- 

 veloped. 



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 digestive system of the Reptilia possesses few characters 

 of any special importance, except that the rectum opens, as 

 in Amphibia, into a common cavity or " cloaca," which not 

 only receives the faeces, but also serves for the discharge of 

 the products of the urinary and generative organs. 



The heart in the Reptiles consists of two completely separate 

 auricles, and a ventricular cavity, which is divided into two by 

 an .incomplete partition. In \hzCrocodilia alone is the septum 



