12 FOSSIL TURTLES OF NORTH AMERICA. 



do not. I he phalangeal formula of the first three digits is 2, 3, 3, as in Emydida?. 

 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 Cheloniidae, but the opening representing 

 the united ischio-pubic foramina is larger. The ilium is short and slender and 

 the upper end is turned backward. 1 he ischia have short posterior processes. 



The femur (plate 3, figs. 6, 7) is slightly 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 is 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 S1DE-NECKT TURTLES. 



It is necessary to make some observations on the osteology of the members of 

 another superfannly, 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 Pelusios (Sternotheerus) 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 ' Ba'ena; 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 ilium. 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. In the Pleurodira there is probably always an intergular scute 

 present. Sometimes the gulars meet in front of it. 



The cervical vertebne 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 vertebrae is given from the neck of 

 Hydromedusa tectifera. 



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 



