EOCENE TESTUDINATA. 
43 
TESTUDINATA. 
The genera of Tortoises obtained number six. Of these, Trionyx, 
Bermatemys, and Tmys still exist, while Plastomeniis, Baena, and Hadrianus 
are extinct, and, so far as known, characteristic of the Eocene period alone. 
The entire number has been found in the Bridger beds of Wyoming. 
Besides the descriptions from the carapace and plastron given below, 
mention may be made of some vertebrae whose correct reference cannot 
now be made. These are opisthocoelian, with well-developed zygapophyses, 
and a solid mass of a depressed-oval form for neural spine. The centra are 
compressed and elongate, and indicate a long tail, as in the genus CJielydra. 
The diapophyses are well developed, and the chevron-facets are excavated 
and continuous with the posterior cup. 
TRIONYX, Geofifr. 
Turtles of this genus were exceedingly abundant during the Eocene 
period in New Mexico. Over considerable tracts, where other fossils are 
rare, these are commonly found. They present considerable uniformity of 
sculpture, and mostly have the coarse character of the Cretaceous species 
rather than the delicate patterns of surface of the existing and many of 
the Miocene species. The only species which appears to be identical with 
those of the Bridger formation is the T. guttatus, Leidy. 
All of the species are of considerable size, and the bones of the cara¬ 
pace and plastron are rather thick. The sculpture of the median portion 
of the carapace is in all a coarse, uninterrupted honeycomb; that of the 
plastron is of finer character. To T. leptomitus, Cope, I refer five individuals; 
to T. ventricosus, two individuals; to T. cariosus, Cope, five individuals; to T 
radidus, Cope, three; and to T. guttatus, three. Many other specimens are 
not finally classified, owing to the absence of the essential parts of the 
carapace, &c. 
The characters are as follows: 
I. Costal bones with transverse ribs at their distal ends: 
A. The dermal ossification extending beyond the bases of the free rib-ends. 
