PHYLOGENY OF SHELL OF TESTUDINATA 423 



cals may be modified to suit the requirements of the creatures, 

 and we need not suppose that these modifications required great 

 periods of time. If indeed Pyxis has all of its cervicals procoe- 

 lous, we can hardly conclude that its ancestors back to the primi- 

 tive turtles had such cervicals. 



However the ancestors of Dermochelys took their origin, the 

 neck was, and has probably always been, short. If they formed 

 one of the two divisions resulting from the first cleavage of the 

 order they may have very early taken to a habitual aquatic 

 existence. Leading such a Hfe and possessing short necks, it is 

 improbable that they would have developed side-bending necks. 

 Having the same number of cervicals, each composed of the same 

 primary elements, and experiencing the same needs in sustaining 

 the head in swimming and in protecting it as did the Cheloniidae, 

 there seems to be no reason why exactly the same kind of cervi- 

 cals should not have been produced. If shght differences at 

 first existed, we must suppose that these would have been eUmi- 

 nated in time, unless we believe that heredity prevailed over 

 adaptability. 



It will be impracticable to consider all of the seven structures 

 which Doctor Versluys discusses as showing a probable close 

 relationship between Dermochelj^s and the Cheloniidae. One 

 may grant that his arguments possess force, without admitting 

 that they subvert other considerations. Some of the structures, 

 as the intertrabecula and the pouches in the nasal passages, are 

 of obscure origin and purpose and in need of further investiga- 

 tion. As regards the intertrabecula, may it not have been 

 possessed by the protestudinates and transmitted by them to 

 the Thecophora and the Athecae ahke? It may later have been 

 lost by most members of the former group. Relatively few 

 testudinates have been examined for this structure, and the 

 discovery of it in any one species of Cryptodira outside the 

 Cheloniidae, in any of the Pleurodira, or of the Trionychoidea 

 would be fatal to the conclusion that has been drawn from its 

 presence in the Cheloniidae and Dermochelyidae. 



As to the structure of the roof of the mouth, the palatine bone, 

 and the position of the choanae, one might easily admit all that 



