422 OLIVER p. HAY 



it may be said that the primitive testiidinates had relatively 

 undifferentiated cervicals and short necks which could be bent 

 equally well in all directions. A retraction of the head for de- 

 fense, first between the fore legs and later into the shell by bend- 

 ing the neck in a vertical plane, is the action that has been adopted 

 by the great majority of turtles, not only the Cryptodira, but 

 also the Trionychoidea. The method of protecting the head 

 resorted to by the Pleurodira is a special one, and must have 

 been the result of special conditions. Of H\ang species of turtles 

 about four-fifths bend the neck in a vertical plane, only one-fifth 

 in a horizontal. Of known extinct species apparently many 

 more than four-fifths belong to Cryptodira and Trionychoidea. 

 Hence if Dermochelys was derived from an independent branch 

 of the protestudinates, there are certainly more than four chances 

 out of five that the species would have adopted the habit of 

 bending the neck in a vertical plane. 



To Versluys' second proposition one may reply that it is not 

 true that the forms and the order of succession of the cervicals 

 are as fixed in the Cryptodira, as might be supposed from his 

 statement. The reader may consult Vaillant's paper on this 

 subject (Ann. Sci. Nat., ser. 6, vol. 10, art. 7, pp. 1 to 106, pis. 

 25 to 31). 



Variations in the form of the articular surfaces are found in 

 the cervicals of other groups of turtles. In the pleurodires 

 they are constructed so as to permit easy movement in a hori- 

 zontal plane; but there exist deviations from the general plan. 

 In the Trionychoidea (essentially Cryptodira) the typical ar- 

 rangement is for all except the first and last to be convexoconcave. 



Probably no one is able to say what advantages result to the 

 cryptodires in having the fourth so generally biconvex, with 

 those in front of it convexoconcave and those behind it concavo- 

 convex. Versluys (p. 325) has suggested that it is in adapta- 

 tion to the strong curvature of the neck during retractions of 

 it; but in the trionychids the curvature is excessive, and here 

 all the vertebra, except the first and the last, are convexoconcave. 

 That it is a matter of indifference one can hardly beheve. We 

 seem to be justified in concluding that the forms of these cervi- 



