GENERAL CHARACTERS OF REPTILES. 527 



pieces which enter into its formation are nine in number ; and, 

 instead of being placed all in a row, the one after the other as 

 in Mammalia, they are, with the exception of a single one, 

 ranged in pairs, and soldered or articulated together, so as to 

 form a large oval plate. Sometimes this shield is entire and 

 solid through all its length ; sometimes it is divided into three 

 parts, of which the anterior and the posterior are slightly mov- 

 able ; and in other cases, again, it is hollowed out in the centre 

 like a frame. It is fixed on each side to the carapace, either by a 

 large bony prolongation, or by cartilages ; and, in front as well 

 as behind, an interval is left between the two, in order to allow 

 the head, the members, and the tail to pass out. The carapace 

 and plastron, as we have already said, are only covered by the 

 skin ; there is no muscle inserted on the external surface ; and 

 it is consequently in the interior of the trunk, that those of the 

 neck and members are fixed. The shoulder, instead of being 

 supported upon the external surface of the walls of the thorax, 

 is lodged in the interior of this cavity ; and the pelvis is, so to 

 speak, drawn up within the abdomen. 



466. The bones of the shoulder (0, cl> co) are articulated with 

 the vertebral column, at one end, and with the sternum, at the 

 other ; so as to form a kind of ring between the carapace and 

 plastron. We there distinguish three branches, frequently 

 united together at an early period, and converging towards the 

 articular cavity of the humerus, which is formed by their meeting. 

 One of these bones (0), suspended to the vertebral column, is 

 evidently the scapula ; the second (co\ which is directed back- 

 wards, is analogous to the coracoid lone of Birds ; and the third 

 (cl), which descends to unite with the plastron, is the represent- 

 ative of the clavicle. The pelvis bears a great resemblance to 

 the ring formed by the bones of the shoulder. It is composed of 

 three pairs of distinct pieces ; an iliac bone, which is attached 

 to the transverse processes of the posterior vertebrae of the cara- 

 pace ; a pulls, and an ischium, which are both directed towards 

 the plastron, and are united to their fellows. 



467. In other Reptiles, the form of the bones of the shoulder 

 bears a greater resemblance to that which we have already seen 



