The Fossil Turtles of the Bridger Basin— Hay. 339 
Professor Cope referred to the genus Plastomenus 
some fragmentary materials from the Cretaceous, but there 
is no certainty about the correctness of the conclusion. 
Nevertheless, we must suppose that there were Cretaceous 
Plastomenidse ; and the writer believes that it is from this 
family that the Trionychidse have been derived. The 
plastomenids are nearer to the Cryptodira, and have through 
degeneration, lost their peripheral bones. Through still 
further degeneration, in adaptation to aquatic life, the cara- 
pace and especially the plastron of the Trionychidse have 
become still further reduced. The Plastomenidae possess a 
preneural bone, and from them the genus Aspideretes re- 
tained the bone. 
Of the Anosteiridse we know too little to permit any 
attempts at ascertaining their history. 
As regards the Dermatemydidae, represented by Bapte- 
mys, it seems to the writer that we are justified in connect- 
ing the family with Adocus, which is a common genus in 
the Upper Cretaceous of New Jerse3\ One species of the 
genus occurs in the Puerco beds of New Mexico. Professor 
Cope's Chelydra crassa {Hoplochelys crassd) of the Puerco 
appears to be a dermatemyd. 
Of the Emydidae no species has been described from any 
formation older than the AA^asatch ; but from this Cope has' 
described on good materials a number of species. At 
almost the same time, species appeared in the London Clay 
of England, and these have been referred to the existing 
genus Chrysemys. No older true emyd has been described 
from any country; but in the American ]\Iuseum of Natural 
History there is a specimen the relationships of which ap- 
pear to be with the Emydidae. It comes from the Judith 
River beds of Montana, and was collected many years ago 
for Cope. The plastron is broad and appears to be con- 
structed as in the Emydidae. If further investigation and 
discovery shall confirm this reference of the species to the 
Emydidae, our country may for awhile claim the origin of 
the family. 
As already stated, the Testudinidae, or true land tor- 
toises, are represented in the Bridger beds by the genus 
Hadrianus, which affords at least one large species. The 
