20 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 wice 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 latiremts 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 vertebra. 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 1s 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 vertebra, but to the lengthening of the individual vertebra. 
The most important modifications of the cervical vertebra are to be found, not in 
their mere elongation, but in the structure of their parts. Originally, as we learn 
from Glyptops plicatulus, the vertebral centra were all biconcave. In Baéna and 
Chisternon 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 vertebre 
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 Athece, 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. 
