524 GENERAL CHARACTERS OF REPTILES. 



recognised as being analogous to them. The skull is always 

 small, and the face lengthened ; the lower jaw is composed of 

 several pieces, as in Birds ; and is articulated to a bone distinct 

 from the temporal (the os quadratum, or tympanic bone, 326). 

 Sometimes even this bone is, in its turn, suspended to a mov- 

 able lever ; an arrangement which greatly increases the dilata- 

 bility of the mouth, as we shall presently see when speaking of 

 the deglutition of Serpents. The upper jaw is generally im- 

 movable ; but in Serpents it is articulated so as to execute cer- 

 tain movements. Amongst several reptiles, Lizards and Tor- 

 toises for example, the bones of the cranium are prolonged 

 laterally over the temples, in the manner of a shield, and thus give 

 to the head a considerable length. Lastly, the head is in gene- 

 ral but slightly movable ; and is articulated upon the vertebral 

 column by means of a single condyle. The bones of the trunk 

 present, in their arrangement and number, more considerable 

 variations. In Lizards, Crocodiles, and other Reptiles formed 

 on nearly the same plan, there are generally but few anomalies 

 to be remarked ; and we shall only notice, that the ribs are more 

 numerous than in Mammalia or Birds ; and that they protect 

 the abdomen, as well as the thoracic portion of the body. 

 Amongst Serpents, the sternum is wanting, as well as the bones 

 of the members ; and the ribs, whose number is very great, are 

 free at their lower extremity : we sometimes count more than 

 300 pairs of them, in the Colubers for example ; and they are so 

 movable, that the animal employs them as supports in its 

 crawling motion. The vertebrae also possess great mobility ; 

 and are articulated together by means of a rounded eminence on 

 each, received into a corresponding cavity in the next, and held 

 by the aid of ligaments ; so as to form a series of ball-and- 

 socket joints (ANIM. PHYSIOL. 629). But it is in the Tor- 

 toise tribe that the conformation of these bones is the most 

 remarkable ; for they form two great shields, between which the 

 animal can generally withdraw itself completely. One of these 

 shields occupies the back, and is named the carapace ; the other, 

 situated under the body, is called the plastron. (Fig. 317.) 

 They are united together on each side, in such a manner as to 



