336 CHELONIA 



CHAP. 



or spine-like edges of the plates, but by the right and left halves 

 of the plates being actually bent at an angle. This is most con- 

 spicuous at the sides of the shell where it passes into the ventral 

 portion. The latter has two pairs of lateral and one median 

 ridge. The wliole shell has consequently twelve ridges. The 

 mosaic plates are deeply imbedded in the cutis, being externally 

 as well as internally covered or lined with dense leathery skin. 

 The epiderm is thin, and shows no indications of horny scales. 

 In young specimens the whole shell is soft and very imperfectly 

 ossified, later on it is quite rigid, although comparatively thin. 

 It is nowhere in contact with the internal skeleton, except by 

 a nuchal bone, which by a descending process articulates with the 

 neural arch of the eighth cervical vertebra. 



The affinities of the Sphargidae and their position in the 

 system are still debatable. Whilst some authorities, e.g. Cope, 

 Dollo, and Boulenger look upon Sphargis as the sole remnant 

 of a primitive group in opposition to all the other recent 

 Chelonia, Baur considered it the most specialised descendant of 

 the Chelonidae. Dames agreed with him. Van Bemmelen has 

 modified this view in so far as he regards Sphargis as the most 

 specialised Chelonian, but considers the differences between it and 

 the Chelonidae great enough to conclude that both Sphargidae 

 and Chelonidae represent two independent, partly parallel, 

 branches whicli have arisen from two different groups of 

 terrestrial tortoises. Case,^ from the study of Protostega and 

 other fossil forms, tends towards Baur's view. He believes 

 that Sphargis is the culminating form of a branch which 

 through Pscphophorus and with Uosj^hargis has sprung from some 

 creature like Lytoloma, whicli at the same time is the starting- 

 point of another branch which culminates in the genera 

 Thalassochelys and Chelone, while lastly a third branch contains 

 Protostega, Protosphargis, and Pseudosphargis. In other words, he 

 considers them all Chelonidae. If he is right we have of course 

 no business to separate Sphargis with its fossil allies from the 

 rest of the Chelonia as " Athecae." 



However, Case has not proved his point. It is easy enough 

 to understand that the characters of the cranium and plastron 

 of Sphargis are in a condition which by partial reduction can be 

 derived from that of typical Chelonidae. The structure of the 



' Journ. Morph. xv. 1897, p. 21. 



