336 The American Geologist. J"°«' i»o^ 
eight pairs of costal plates and no preneural. Many of the 
living species belong to this genus. A. uiniaensis (Leidy) 
A. radulus (Cope), A. scutumantiquuDi (Cope), and prob- 
ably A. concentricus (Cope), all belong here. All were 
described under the genus Trionyx. A. radulus was orig- 
inally described by Cope from the Wasatch of New Mexico, 
but he referred to the species also a specimen from the 
Bridger of Wyoming. This is now in the American Mu- 
seum of Natural History. The type from New Mexico ap- 
pears to be lost. The American Museum possesses two 
carapaces, one nearly complete, collected near Opal, Wyo- 
ming, which certainly belong to the same species as Cope's 
Wyoming individual, identified as A. radulus; but whether 
or not the Wasatch specimens are the same remains doubt- 
ful. The Opal specimens come from the base of the Bridger 
beds ; and it is possible that the species continued through 
the Wasatch and Green River epochs to the Bridger. 
To the genus Amyda is referred with some doubt a fine 
skull which the writer discovered near the mouth of Cot- 
tonwood creek, and to which the name A. tritor has been 
given (Science, xix, 1904, p. 254). As the shell was not 
present, the skull may belong to some other genus and 
possibly to some of the already described species. It has 
a length of nearly six and one-half inches and resembles 
closely the skull of Platypeltis ferox of our southern states. 
The genus Platypeltis is characterized by the reduc- 
tion of its costal plates to seven pairs. At the most, there 
may be vestiges of the eighth costals. The young have a 
smooth or finely granulate skin. The living species of 
North America belong to this genus. Four species are 
known from the Bridger. One of these is P- heieroglyptus, 
described as Trionyx heteroglypius by Cope from the 
hinder half of the carapace. The nuchal was present with 
Cope's specimen, but not known to him. Another and 
complete carapace was collected in 1903. In Cope's type 
the eighth costals were wholly undeveloped ; in the new 
specimen these are present, each lying in a notch in the 
posterior border of the corresponding seventh costal. 
Platypeltis trionychoides (Cope) was described from frag- 
mentary materials as a Plastomenus. One nearly complete 
