SIDE-NECKED TORTOISES. 



89 



of S-necked or Cryptodiran tortoises, but since there are also certain features 

 by which the skulls and shells of the two groups can be identified, it is im- 

 portant that these should be noticed. As regards the skull, this is distinguished 

 in the first place by the tympanic ring surrounding the aperture of the ear being 

 complete, as may be seen by comparing the accompanying figure with the one on 

 p. 47, and also by the circumstance that the lower jaw articulates by means of a 



LOWER AND SIDE VIEWS OF SKULL AND UPPER AND LOWER ASPECTS OF LOWER JAW OF 



greaved tortoise. (From Gray, Proc. Zool. Soc., 1870., 



knob-like condyle with a corresponding cavity in the quadrate-bone, whereas in the 

 preceding group the positions of the condyle and cup are reversed. The shell, 

 which is always fully developed and forms a solid box, presents the peculiarity that 

 both the carapace and the hinder 

 part of the plastron are immovably 

 welded to the bones of the pelvis; 

 its upper and lower moieties thus 

 having a bond of union which is 

 totally lacking among the S-necked 

 tortoises. Further, the vertebrae of 

 the neck are furnished with pro- 

 jecting lateral or transverse pro- 

 cesses, which are absent from the 

 latter group. 



In addition to these absolutely 

 characteristic features, there are cer- 

 tain other points connected with the anatomy of the side-necked tortoises which 

 demand a brief notice. With the exception of one species, which lacks horny 



FRONT PORTION OF THE PLASTRON OF A SIDE-NECKED 

 TORTOISE, WITH THE HORNY SHIELDS REMOVED. 



The thick lines indicate the boundaries of the shields. 



