THE SIDE-NECKED TORTOISES 



2447 



FRONT PORTION OF THE PLASTRON OF A SIDE- 

 NECKED TORTOISE WITH THE HORNY 



SHIELDS REMOVED. 

 The thick lines indicate the boundaries of the shields. 



spoken of as the side-necked tortoises, or Pleurodira. This characteristic is alone 

 amply sufficient to separate the group from the foregoing assemblage 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 

 important that these should be no- 

 ticed. 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 

 preceding figure with the one 

 on p. 2404, and also by the circum- 

 stance that the lower jaw articulates 

 by means of a 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 projecting lateral or trans- 

 verse processes, which are absent from the latter 

 group. 



In addition to these absolutely characteristic 

 features, there are certain 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 shields on the shell, the 

 whole of these tortoises are characterized by the pres- 

 ence of an intergular (i.gu~) shield between the two 

 gulars {gu) on the front of the plastron; such in- 

 tergular shield being, as we have seen, but very 

 rarely present in the S-necked group. Very gen- 

 erally among the present assemblage one or more of 

 the pairs of costal bones of the carapace may meet 

 in the middle line, owing to the absence of some 

 of the median unpaired series of bones; in cer- 



RIGHT HALF OF THE CARAPACE OF tain cases the whole of the costals thus meeting, 



owing to the absence of all the neural bones. 

 Whereas, in one family of the group the plastron 

 contains the same nine bones as in the side-necked tortoises, in a second family 

 there are eleven bony elements in this part of the shell/owing to the presence of 



THE BLACK STERNOTHERE WITH 

 THE HORNY SHIELDS REMOVED. 



