528 MANUAL OF ZOOLOGY. 



bone (fig. 288, a) ; and as this often projects backwards, the 

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

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

 ent, 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. 



As regards the exoskeleton, most Reptiles have horny epi- 



b 



Fig. 288. Skull of a Serpent (Python), b Articular portion of the lower jaw ; 

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



dermic scales, and they are divided into two great sections 

 called respectively Squamata and Loricata according as the 

 integumentary skeleton consists simply of these scales, or there 

 are osseous plates developed in the derma as well. In the 

 Tortoises, 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 Jieart in the Reptiles consists of two completely separate 

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

 an incomplete partition. In the Crocodilia alone is the septum 

 between the ventricles a perfect one ; and even in these, as in 

 all other Reptiles, the heart consists ftmctionally of no more 

 than three chambers. The ordinary course of the circulation, 

 where the ventricular septum is imperfect, is as follows : The 

 impure venous blood returned from the body is, of course, 

 poured by the venae cavas into the right auricle (fig. 289, a), and 



