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 
che along the median line. As shown by Baur (Jour. Morphology, tv, 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 
Baéna 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. 1, it will be seen that the temporal region is 
nearly as completely rooft over as is that of Lepidochelys (plate 1, fig. 1). There 
are no nasal bones and the supraoccipital spine is long. In one feature of the 
temporal roof this turtle is different from Leprdochelys. 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 premaxillz are 
of considerable size. The vomer is absent. The triturating surface of the maxilla 
is broad and furnisht with ridges. The choanz 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 DERMOCHELYID/E. THE LEATHERBACK TURTLES. 
It is necessary now to describe in brief terms the skeleton of the leatherback 
(Dermochelys cortacea), 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 Muséum d’Histoire 
Naturelle de Paris, volume vit, 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 
