126 REPTILES 



forming one unbroken case all round, which is composed of 

 many hundreds of little bony plates, irregularly polygonal, 

 fitting closely into each other with their sutural edges, and 

 giving the shell a beautiful mosaic appearance. On the dorsal 

 side are a median row and three pairs of lateral rows of larger 

 plates, and these form seven longitudinal blunt ridges which all 

 converge towards the triangularly pointed tail-end of the shell. 

 . . . The mosaic plates are deeply embedded in the cutis [skin], 

 being externally as well as internally covered or lined with 

 dense leathery skin. The epiderm is thin and shows no indi- 

 cation of horny scales. In young specimens the whole shell is 

 soft and very imperfectly ossified, later on it is quite rigid, al- 

 though comparatively thin. It is nowhere in contact with the 

 internal skeleton, except by a nuchal [nape] bone, which by a 

 descending process articulates with the neural arch of the eighth 

 cervical [neck] vertebra." 



As already mentioned, different views are entertained with 

 regard to the relationship between the shell of the luth and 

 that of typical Chelonia. According to one view, the luth's 

 shell is a degenerate modification from that of the true marine 

 turtles ; according to a second the two have nothing in com- 

 mon, and leathery turtles form a group distinct from all 

 other Chelonia. An attempt to reconcile such opposite 

 opinions has been made by regarding Dermochelys as the 

 most specialised of all Chelonia, but at the same time de- 

 rived from terrestrial forms independently of the Chelonidcs. 

 Another view is that the mosaic plates of the luth correspond 

 to the bony plates of crocodiles, while the plates of the shell 

 of other chelonians are of different type, and correspond (as 

 those of the plastron certainly do) to the category of abdominal 

 ribs. If this be true, the Chelonidce and the Dermochelyida in- 

 dicate independent types of diverse origin. The same must be 

 the case if certain mosaic-like dermal plates from the German 

 Trias described as Psephoderma really belong to an ancestral 

 form of leathery turtle. 



This notice of defensive adaptation in chelonians may be 

 closed by a second quotation from Dr. Gadow's work, dealing 

 with general structural adaptations in the group : — 



" Nearly the whole organism," he writes, "has been altered. 

 The hard, firm carapace has partly rendered the supporting 



