The Fossil Turtles of the Bridger Basin— Hay. 3 3 7" 
carapace and two plastra, one nearly complete, show now 
that the species is a Platypeltis. Platypeltts serialis (Cope) 
was also originally described as a Plastomenus, from the 
Wasatch beds of New Mexico. The American Museum 
party of 1903 collected a carapace and large portions of two 
plastra; and Mr. Paul Miller, of the American Museum, 
collected in 1904, a complete carapace of a species which it 
seems now necessary to refer to Cope's Wasatch species. 
It is not at all unlikely that better Wasatch materials will 
prove the Bridger form distinct. In the complete carapace 
referred to, the costals of the eighth pair lie in notches in 
the costals of the seventh pair and not in contact with each 
other. 
Axestemys byssinus was set apart by Cope in a separate 
genus, Axestus, preoccupied ; but the materials representing 
the only species are scanty and the characters doubtful. The 
plastron is devoid of pits and ridges. Cope made much of 
a clothlike network of the superficial bony fibers ; but this is 
found in all trionychids. 
Most of the species of Bridger turtles which have been 
described have come from the region within a few miles of 
Ft. Bridger, the Grizzly buttes, Cottonwood creek, and 
Church buttes. Of some of the species described by Cope 
and Leidy exact localities are not given. Not enough col- 
lecting has been done in other localities and levels to enable 
us to determine with certainty whether the species changed 
rapidly or endured throughout the whole Bridger epoch. 
Further collecting ought to be done about Opal, in the low- 
est division of the Bridger and along Henry's fork, where 
the higher beds are exposed. 
It is now in order to determine if possible the origin of 
the A^arious parts of the Bridger turtle fauna. 
Considering first of all that important eleixient consti- 
tuted by the genus Baena, we cannot doubt that it de- 
scended from ancestors which inhabited the same region 
during the Upper Jurassic, and which are represented by 
Compsemys plicatula and Probaena sculpta, These species 
were closely related to Pleurosternon and Platychelys of 
Europe and were a part of that fauna which had at that 
time extended itself over the Northern Hemisphere. The 
