1 6 FOSSIL TURTLES OF NORTH AMERICA. 



basisphenoid and the basioccipital; they shut off the latter bones from contact 

 with the quadrate. The inner borders of the pterygoids lie upon the upper side of 

 the basisphenoid. 



In one important respect the leatherback and its few fossil relatives differ from 

 all other turtles. This is in the fact that the parietal bone does not send downward, 

 in front of the exit of the trigeminal nerve, a plate to join the pterygoid. 



The occipital condyle remains cartilaginous. The pedicel of the quadrate is 

 slender and directed strongly forward. The lower jaw is feebly developt and fitted 

 for soft food. The articular remains cartilaginous. The angular and the supra- 

 angular are short, as seen from the outer side of the jaw, but are prolonged on the 

 inner side. The prearticular and the coronoid are not present. 



The parts of the scapula are short and thick. The procoracoid process makes 

 an obtuse angle with the body of the bone. The coracoid bone is about twice as 

 long as the process mentioned, and the free end is somewhat expanded and flattened. 



The humerus is a massive bone and much flattened. The general form is that 

 of the humerus of Lepldochelys, but the radial tuberosity is brought down to a 

 point more than half-way from the head of the bone to the distal end. The end 

 for articulation with the ulna and the radius is very broad, and nearly semicircular 

 in outline. The ectepicondylar passage is a foramen wholly in the bone, about the 

 middle of its width. 



The radius and the ulna are stout bones of about equal length and only about 

 half as long as the humerus. The carpus has 2 bones in the first row, a centrale, 

 5 bones in the second row, and a large pisiform. Sometimes the fourth and the 

 fifth distal carpals are co-ossified. The digits have the normal number of pha- 

 langes; that is, 2 in the thumb and 3 in each of the others. The second, third and 

 fourth digits are greatly elongated, being twice the length of the humerus measured 

 from the head. None of the digits bears a claw. All are bound together in a com- 

 mon envelope of skin to form a powerful flipper. 



The pelvis is in many respects different from that of the Cheloniidae. The 

 form of the ilium and its position with respect to the pubis do not differ greatly 

 from those of the ilium of Caretta, but the ischia are broader fore and aft than in 

 Caretta. The pubes are connected along the midline for a much greater distance 

 than in the Cheloniidae. Posteriorly they approach the ischia, so as nearly to join 

 them, the interspace being filled by cartilage. The ischio-pubic foramina are 

 extremely small and nearly separated in the midline. The lateral processes are 

 very large, extending outward and forward, so that at their extremities the pelvis 

 is twice as wide as it is across the narrowest part of the pubes. Each process 

 terminates in a large plate of cartilage. There is likewise a large spatulate prepubic 

 cartilage. 



The hinder limb is reduced in size, as in the Cheloniidae, and offers differences 

 only in detail. 



