REPTILIA. 189 



jaw articulates with the skull by a quadrate bone (fig. 546, 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 present, but are 

 not sunk in separate sockets or alveoli, except in the Croco- 



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Fig. 546. — Skull of a Serpent {Python) h, Ai-ticular portion of the lower jaw ; a, Quadrate 

 bone ; c. Squamosal portion of the temporal bone. 



diles 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. 



Eibs 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 Eeptiles 

 from the Amphibia, 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 Ccccilicv they 

 are large and well developed. 



As regards the exoskeleton, all Eeptiles have horny 

 epidermic scales, and they are divided into two great sec- 

 tions — called respectively Squarnata and Loricata — accord- 

 ing 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 en- 

 closed. 



