14 FOSSIL TURTLES OF NORTH AMERICA. 



olecranon. The tarsus and digits do not differ enough from those of emyds to 

 require further notice here. 



The pelvis of Hydromedusa differs greatly from that of the emyds and triony- 

 chids. The ilia are much expanded and triangular at their upper ends, and have 

 become joined by a rough suture with the eighth costals. The lateral processes of 

 the pubes are stout and are sutured to rough surfaces on the xiphiplastra. The 

 anterior branch of the pubis is slender and joins its fellow at the midline. The 

 ischia are stout bones, sutured to the hinder border of the xiphiplastra. The lower 

 portion of each ischium expands into a great lateral process, which runs forward 

 and inward to join the one from the other side, the whole length of each process 

 being sutured to the xiphiplastron. There is no bony union of the pubes and the 

 ischia along the median line. As shown by Baur (Jour. Morphology, iv, 1891, p. 

 351) there is, in many Pleurodires, a greatly lengthened prepubic cartilage. It is 

 to be compared with the prolongation forward seen in the pubes of species of 

 Baena and Chisternon. 



The hinder limb does not differ notably from that of the emyds. 



Illustrations are here furnisht of the skull of Podocnemis expansa, another 

 Pleurodire, the structure of which in many respects strikingly differs from that of 

 Hydromedusa. From plate 4, fig. I, it will be seen that the temporal region is 

 nearly as completely rooft over as is that of Lepidochelys (plate I, fig. i). There 

 are no nasal bones and the supraoccipital spine is long. In one feature of the 

 temporal roof this turtle is different from Lepidochelys. In the latter the postfrontal 

 bone extends backward nearly to the hinder border of the roof; in Podocnemis 

 the postfrontal bone is very small and its place in the roof is mostly occupied by the 

 quadratojugal, which rises over the squamosal, excluding the latter from contact 

 with the parietal (plate 4, figs. 1-5). 



Fig. 2, plate 4, represents a palatal view of the same skull. The premaxillae are 

 of considerable size. The vomer is absent. The triturating surface of the maxilla 

 is broad and furnisht with ridges. The choanae are restricted. The palatines are 

 broad and meet throughout their length at the midline. The pterygoids, too, are 

 broad, little separated mesially by the basisphenoid, and the outer border of each is 

 turned upward into a scroll-like process. The articular surface of the quadrate, 

 for the lower jaw, is seen to be concave. 



Fig. 5, plate 4, shows the same skull viewed from the side. 



The lower jaw (plate 4, figs. 3, 4) presents a ball-like articular surface for the 

 quadrate. In front of this are seen the angular, the prearticular, the splenial, and 

 the dentary, the latter consolidated with its fellow at the symphysis and furnisht 

 above with a broad and ridged triturating surface. On the outside of the hinder half 

 of the ramus is the supraangular. 



THE DERMOCHELYlDyt. THE LEATHERBACK TURTLES. 



It is necessary now to describe in brief terms the skeleton of the leatherback 

 (Dermochelys conacea), one of the most extraordinary of turtles. It attains a great 

 size and is the most thoroly aquatic turtle that is known (fig. 7). Figures of all 

 portions of the skeleton may be found on the plates illustrating a paper by Paul 

 Gervais on this turtle, publisht in the Nouvelles Archives du Museum d'Histoire 

 Naturelle de Paris, volume vm, 1872. 



The ribs of this turtle, instead of being consolidated with costal plates that 

 unite edge to edge to form a carapace, are wholly free from one another. The first 

 rib is wholly free from the second, the tenth from the ninth. The only thing that 



