2O FOSSIL TURTLES OF NORTH AMERICA. 



The jaws have suffered extensive modifications. In the less differentiated 

 turtles the cutting-edges of the jaws are low and the triturating surfaces narrow. 

 The cutting-edges may become deep, without increase of the width of the triturating 

 surfaces and vice versa. For an illustration of great width of the grinding surfaces 

 the reader is referred to the species Rhetechelys platyops (Cope). 



The lower jaw becomes modified to correspond with the upper jaw. The rami 

 of that of Toxochelys latiremis are narrow and slender; while those of Erquilinnesia 

 are broad, flat, and fitted for crushing hard objects. 



The vomer, which is always developt in the Cryptodira, the Trionychoidea and 

 some of the Pleurodira, is wholly missing in some forms of the latter superfamily. 



There is great variation in the pterygoids. The primitive condition appears to 

 be essentially that now found in the Cryptodires in which these bones are of 

 moderate width and extend backward so as to exclude the quadrates from contact 

 with the basicranial bones. This condition exists also in the Amphichelydia and 

 the Trionychoidea. In the Pleurodira the pterygoids have become shortened poste- 

 riorly, so as to let the basicranial bones join the quadrates. They have also become 

 expanded, and the outer edge is rolled up in a scroll-like manner. 



The necks of turtles furnish us with many interesting features. There is 

 no doubt that in the early turtles the neck was short, perhaps less than half as long 

 as the dorsal series of vertebrae. Doubtless the lengthening has been brought 

 about to facilitate the prehension of food, but it has had other consequences. In 

 some species of each of the superfamilies of Thecophora the neck is considerably 

 longer than the dorsal series, but it has retained its primitive shortness in Dermo- 

 chelys and the other sea-turtles. The elongation of the neck is never due to any 

 increase in the number of vertebrae, but to the lengthening of the individual vertebrae. 



The most important modifications of the cervical vertebrae are to be found, not in 

 their mere elongation, but in the structure of their parts. Originally, as we learn 

 from Glyptops phcatulus, the vertebral centra were all biconcave. In Baena and 

 Chtsternon we find the beginnings of differentiation in the articular ends of the 

 centra. The highest stage of differentiation is perhaps to be found in species of 

 Testudo. In Testudo radiata, of Madagascar, the second cervical is convexo- 

 concave; the third and the eighth convexo-convex; the fourth, fifth, and sixth, 

 concavo-convex; the seventh, concavo-concave. The joint between the sixth and 

 seventh and that between the seventh and the eighth are elongated from side to side 

 and divided into right and left portions, forming true ginglymoid articulations. 

 Variations of this arrangement are found even within the genus Testudo. Modifi- 

 cations and less differentiated stages are to be found in other families of Cryptodira. 



The necks of the Trionychoidea are usually greatly elongated and the vertebrae 

 are essentially as in the Cryptodira. The Pleurodira do not possess ginglymoid 

 joints, but in at least one genus there are saddle-shaped vertebral articulations, as 

 in birds. 



In the development of the mechanisms permitting flexion of the neck two distinct 

 paths were followed, the Athecae, the Cryptodira, and the Trionychoidea taking the 

 one, the Pleurodira the other. The goal reacht in each case is very different from 

 that attained in the other. In the first case, the neck is bent in a perpendicular 

 plane; in the case of the Pleurodira, in a horizontal plane. As the neck of members 

 of the Cryptodira and Trionychoidea lengthened, the head began to be withdrawn 

 within the shell for protection. In the case of the Pleurodires the neck is bent 

 laterally, and it and the head are protected by the projecting borders of the shell. 

 If the neck is so long that it would carry the head beyond the axilla it is bent first 

 in one direction, then in the other. 



