334 ^Ae American Geologist. •^"°«- ^^^^ 
figured by Siebenrock (Sitzber. Akad. Wissensch. Wien, 
xci, 1902, p. 836). No specimen of Plastomenus has yet 
furnished the epiplastra ; but in the x^merican Museum of 
Natural History are several specimens of /*• thomasii 
which present the hyoplastra, the hypoplastra, and the 
xiphiplastra; while one specimen of P. oedemius has fur- 
nished the entoplastron. These materials show that nearly 
as far forward as the entoplastron, the lateral halves of the 
plastron were as closely joined as in the Emydidae. The 
antero-interior angles of the hyoplastra are truncated, so as 
to form a median notch, but this is nearly filled up by the 
broad entoplastron. The fontanelles at the outer ends of 
the hyo-hyoplastral bones too are nearly closed. The cara- 
pacial disk likewise is at an early age filled out nearly to 
the ends of the ribs. A well-preserved skull of Plastomenus 
thomasii was found in 1903, but it does not appear to be 
essentially different from that of the Trionychidae. The 
palate has not yet, however, been exposed to view. The 
skull is long and pointed, as in Platypeltis inutica. The 
limbs afe as yet unknown. 
Of Plastomenus Cope had only fragmentary materials 
when he described the genus ; and, while he recognized that 
the plastron was more complete than in the Trionychidae, 
he was unable to define well the genus. Moreover, he in- 
cluded in the genusi some species which belong elsewhere. 
First from a specimen collected for Prof. Cope in the 
Bridger beds of the Rattlesnake range, Wyoming, but never 
studied b)'- him, was the writer able to understand the real 
structure of the plastron and carapace. A figure of the plas- 
tron of this specimen is here presented. 
