16 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 Lepidochelys, 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. ‘he 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 Ree 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 Cheloniide. 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 Cheloniidz. 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 1 in the reatne’ 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 Cheloniidz, and offers differences 
only in detail. 
