10 FOSSIL TURTLES OF NORTH AMERICA. 
midline in front, there is a deep notch, filled in life by the prepubic cartilage. The 
lateral processes are broad. The ischia are without posterior lateral processes, and 
they are not connected with the pubes on the midline by bone; thus the ischio- 
pubic foramina unite into one in the prepared skeleton. 
The femur, which in the emyds is longer than the humerus, 1 is, in the sea-turtles, 
shorter than the latter element. The shaft is not so straight as is that of the humerus 
and the head is not so much deflected toward the axis of the shaft. “The trochanters 
differ considerably in size and the digital fossa does not descend far between them. 
The tibia and the fibula are relatively stout bones. While in the emyds described 
above, the two upper segments of the fore limb are considerably shorter than the 
corresponding segments of the hinder, the reverse is true in the Cheloniide. Figs. 
5 and 6 of plate 2 represent the femur of Lepidochelys. 
The hinder foot of sea-turtles is, compared with the fore foot, greatly reduced. 
In Caretta the hinder foot, including the tarsus, is only about half as long as the 
fore foot. There are 2 bones in the first row of the tarsus, 5 in the second row. 
THE TRIONYCHOIDEA. THE SOFT-SHELLED TURTLES. 
We will give our attention now to members of another superfamily, the din= 
ony choidea, popularly known as soft-shelled turtles, sometimes as mud-turtles, or 
river turtles. ‘Vhe two last-mentioned names are, however, often applied to other 
groups of turtles. 
In these, as in other turtles, there is a carapace and a plastron; but they differ 
greatly in appearance from those of the emyds. The whole body is greatly deprest. 
For illustrations of these portions of the shell the reader may consult the figures of 
the fossil species on succeeding pages. The carapace is usually composed of neurals, 
7 or 8 in number, 7 or 8 pairs of costals, and a nuchal bone. Only in one genus, 
Trionyx (Emyda Gray), including two Asiatic species, are there any suggestions of 
peripherals. ‘The costal bones are closely articulated to their neighbors and the 
contiguous neurals; but often these costals do not extend to the eae of the ribs; 
or, if they do, it is only in very advanced age. In life the periphery of the shell is 
surrounded by a rim of dense and flexible connective tissue, terminating all round 
in a sharp edge. It is this rim that has suggested the name soft-shell. 
The whole upper surface of the carapace is ornamented with a network of 
ridges which inclose pits of various forms. The size and arrangement of the ridges 
and pits vary in the different species. There are never any traces of horny scutes, 
the outer skin always remaining soft. 
The plastron in some respects resembles that of the sea-turtles, inasmuch as 
there are median and lateral fontanels and the connection with the carapace 1s 
a ligamentous one. The hyoplastra and the hypoplastra are furnisht with median 
and lateral digitations. Of the median digitations one belongs to each hyoplastron 
and is directed forward and inward, to come into contact with the entoplastron. 
Two belong to each hypoplastron, one being directed toward that of the opposite 
side, the other toward the xiphiplastron. Of the lateral digitations one set is 
directed from each hyoplastron forward and outward, the other from the hypo- 
plastron backward and outward. The hyoplastron and the hypoplastron of each 
side are closely sutured together, in some cases co-ossified. 
The entoplastron is V- shaped, the apex being directed forward, an arm resting 
against the hyoplastron of each side. The epiplastral bones are slender and curv ed, 
usually not in contact with each other at the midline. They differ from these 
bones in all other turtles in being excluded from contact with the hyoplastra by 
the arms of the entoplastron. 
