CLASS OF BEPTILES, 



351 



(cs) which articulate with the extremity of the ribs, and 

 which form in some measure the border of the carapace, 

 evidently represent the sternal portion of these ribs, which in 

 mammalia remain cartilaginous, but which in birds are com- 

 pletely ossified. In some tortoises they remain cartilaginous, 

 and in almost all these animals several of them are sup- 

 ported laterally on the edges of the plastron or ventral 

 plate. 



The plastron is formed by the 

 sternum, which presents an extraor- 

 dinary development, and extends 

 from the base of the neck to the 

 commencement of the tail. The bon es 

 composing it are nine in number, 

 arranged in pairs, and so articulated 

 with each other as to form a great 

 oval plate. The plastron is some- 

 times entire and solid throughout its 

 whole extent, sometimes divided into 

 three portions, of which the anterior 

 and posterior are a little moveable ; 

 at other times it is hollowed out in 

 the centre like a picture-frame. 

 Finally, it is fixed on each side to 

 the carapace by bone or by cartilage. 

 All the muscles and soft parts, as 

 well as the organs, are contained within these two plates. 



The bones of the shoulder (o, cl, co) articulate with the 

 vertebral column on the one hand and the sternal on the other. 

 One of these bones (o) suspending it to the vertebral column is 

 evidently the scapula ; a second, directed backwards (<?o), cor- 

 responds to the coracoid bone of birds ; and the third (cl) 

 represents the clavicle, or at least the acromial process of 

 the scapula, with which it generally articulates. 



The pelvis (b) greatly resembles the osseous girdle formed 

 by the bones of the shoulder ; it is also composed of three dis- 

 tinct pairs of bones : the iliac bone, attached to the transverse 

 processes of the posterior vertebrse of the carapace, a pubis, 

 and ischium directed towards the plastron, and united to the 

 corresponding bones of the other side. 



459. In other reptiles, the bones of the shoulder more 

 resemble those of birds. The limbs generally have nothing 

 remarkable ; sometimes they are truncated at the extremity, 



Fig. 348. The Common 

 Land Tortoise (Testudo 

 Grseca), as seen from 

 below. 



