EOCENP] TESTUDINATA. 
47 
the nuchal was large and thick, and tliat it underran the costal, forming a 
beveled suture. Costal capitula wide. The costal bones are scarcely 
thicker distally than proximally, i. e., from 7 to 9 millimeters. 
I originally referred these turtles to the T. uintaensis of Leidy, but now 
believe that they present rather the characters of the nearly-allied T. gutta- 
tus of the same author. Both have been found in the Bridger beds with 
the T. lieteroglyptus and T. concentricus of the writer. 
PLASTOMENUS, Cope. 
U. S. Geological Survey of the Territories, 1872, p. 617. 
The structure of the skeleton in this genus remains incompletely known 
in spite of the abundance of specimens which I procured in the Eocene 
beds of New Mexico. As already stated, it is allied to the genus Trionyx, 
but dilfers in some important points in the bones of the plastron. The 
hyosternal bones which I have seen in P. multifoveatus and P. trionychoides 
are generally like those of Trionyx, while the hyposternals, if I have cor¬ 
rectly identified them, differ materially. These elements are preserved in 
the two species named and in P. corriigatus, and here they display a trans¬ 
verse width behind the inguinal region more like an Emydoid than a 
Trionychoid genus. The inguinal border is thickened, and at the bridge 
somewhat recurved. In P. corriigatus, there is a fontanelle at the supposed 
postabdominal suture, as in Trionyx, while there is no indication of one in 
the P. trionychoides. The hyosternals also display a more completed ossifi¬ 
cation than in Trionyx, in the fullness of the borders between the internal 
and external digitations. Thus, in P. midtifoveatus, the internal border is 
regularly convex, and the processes for the episternal bone scarcely project 
beyond it. The external digital process projects more extensively, while 
the free ends of the ribs extend little or not at all beyond the border of the 
carapace. Among the various remains from Wyoming and New Mexico, 
no marginal bones have been found. 
Portions of the skeletons of the species of this genus are very abundant 
in the Eocene of New Mexico. Though one seldom obtains an entire 
carapace or plastron, the form, size, and sculpture indicate that the remains 
belong to several species. The figures, composed of ridges, pits, etc., 
