12 FOSSIL TURTLES OF NORTH AMERICA. 
do not. The phalangeal formula of the first three digits is 2, 3, 3, as in Emydide. 
The fourth may have 4, 5, or 6, while the fifth finger may have 3 or 4 phalanges. 
‘The pelvis resembles much that of the Chelonide: but the opening representing 
the united ischio-pubic foramina is larger. ‘The ilium is short and slender and 
the upper end is turned backward. ‘The ischia have short posterior processes. 
The femur (plate 3, figs. 6, 7) is shghtly longer than the humerus, which it 
resembles considerably. It may be distinguisht by its having no ectepicondylar 
groove and by the narrower, more elongated, head. The trochanters are wide 
apart and wholly separated by the interdigital fossa. 
The tibia is a stout bone nearly three-fourths as long as the femur. As in other 
turtles, the tibia is a slenderer bone. ‘The hinder foot 1s still more elongated than 
the anterior, that of P. spinifera being more than a third longer than the femur. 
The first three digits have the same number of phalanges as other turtles; that is 
2, 3, 3.. The fourth digit may have 4 or 5 phalanges; the fifth, 2 or 3. The first 
three have claws. 
THE PLEURODIRA. THE SNAKE-NECKT, OR SIDE-NECKT TURTLES. 
It is necessary to make some observations on the osteology of the members of 
another superfamily, the Pleurodira. The shell is composed of the same elements 
as in the emyds and often presents no important differences from that of the latter. 
There are genera in which the neurals are reduced in number and a few in which 
these bones have been wholly supprest. In a few living genera and in some 
that are extinct there is present a pair of bones, unknown in the Cryptodira and 
Trionychoidea, the mesoplastrals, interposed between the hyoplastrals and the 
hypoplastrals. In Pelustos (Sternotherus) these bones join across the plastron. 
In Pelomedusa they are small triangular bones occupying the middle of the bridge 
only. The same bones occur in the species of Baéna; and the reader may consult 
the figures under that genus. 
The shell of all Pleurodira differs from that of other turtles in forming sutural 
connections with the pelvis. The eighth costal plates develop each a sutural surface 
for the upper end of the illum. On each of the xiphiplastrals are two sutural scars, 
the anterior for union with the pubis, the posterior for union with the ischium. 
On other pages will be found figures of the species of Taphrosphys, which represent 
these articulations. Inthe Pleurodira there is probably always an intergular scute 
present. Sometimes the gulars meet in front of it. 
The cervical vertebrz are, as in all turtles, 8 in number, but they differ greatly 
from those of the Cryptodira and of the Trionychoidea. In contradistinction to the 
neck of the latter turtles, that of the Pleurodira is constructed for free flexure in a 
horizontal plane. This is effected by having the centra joined by ball-and-socket 
joints and by having the zygapophyses of the two sides placed close together and 
high above the centra. A description of these vertebra is given from the neck of 
H ydromedusa te ctifera. 
The neck is nearly a third longer than the dorsal series of vertebrae. ‘The first 
cervical, unlike that of the other superfamilies, has all the elements consolidated 
and is two-thirds as long as the longest. As regards the articular ends of the centra 
we have the following: The first and the seventh are concavo-concave; the second, 
third, and fourth are convexo-concave; the fifth and the eighth are convexo-convex; 
the sixth is concavo-convex. All the cervicals possess well-developt transverse 
processes, with broad bases. Along the lower side of the centrum of each runs 
a sharp crest. The postzygapophyses of the first two vertebra are separated by 
