1883. | The Extinct Rodentia of North America. 47 
mys gervaisi of the French Eocene. Bravard briefly dis- 
tinguishes the genus as distinct from Arctomys in the greater 
thickness of the angles of the molars, which thus become tuber- 
cles. Only the mandible and mandibular teeth of the P. ger- 
vatsu are known. It has been found in the Upper Eocene, near 
Perreal, Apt, France. 
I have seen six species of this genus, of which two, P. hians 
Cope, and P. undans Marsh, belong to the Bridger beds only; 
one P. leptodus Cope, to the Washakie; one P. duccatus Cope, 
to the Wasatch and Wind river, and two, P. delicatior Leidy, and 
P. delicatissimus, Leidy, to all the Eocenes except the Washakie. 
The following comparison of the P. delicatissimus with the Sciurus 
niger, or common gray squirrel, may be made. The pelvis is longer 
as compared with the bones of the fore leg. The humerus is longer 
as compared with the length of the ulna and radius. The species 
exceeds the S. ziger in size, one-fourth linear. 
It is then probable that the species of this, the oldest known 
genus of Rodentia, were arboreal, like the squirrels‘’of the present 
geological period. 
SYLLOPHODus Cope. 
This genus is much like Theridomys of the European Upper 
Eocene and Lower Miocene, and may be the same. The species 
were smaller than those of the last described, and are only known 
from lower jaws. These contain teeth which differ from those 
of Plesiarctomys in having cross-crests which are slightly con- 
nected at one side. They look like the unworn condition of 
Ischyromys, of which genus they may be the ancestor. Two 
species, S. minimus and S. fraternus have been described by Leidy. 
Both are from the Bridger horizon. 
MIocENE RODENTIA. 
Iscuyromys Leidy. 
The essential features are, dentition, I.,; C., $8; M., į; the molars 
with two crescents on the inner side above, each of which gives 
rise to a cross-ridge to the outer margin. In the mandibular 
series the crests and crescent have a reversed relation. No 
cementum. 
To the above characters given by Dr. Leidy, I have added the 
absence of postfrontal processes, and the superior position of the 
infraorbital foramen. Also that the pterygoid fossa is large, and 
