1883. | The Extinct Rodentia of North America. 53 
are most complex with reticulate 
ridges. (Fig. 8, c.) 
There is a suggestive resemblance 
“between the forms of the molar teeth 
of the Meniscomys hippodus and those 
of the Haplodontia rufa now living 
in Oregon. The two genera have 
doubtless had a common origin, but 
the present. differences are consider- 
able. Thus the Haplodontia has an 
extended osseous cavum tympani, 
which does not exist in Menisco- 
"mys, 
CASTOR Linn, 
G. 8.—a-é, superior molars af 
The beaver is the largest rodent Memscomy iolophus, — é, 
inferior mola M. nitens from 
of the northern hemisphere, and has iey ska aes a. skull of the Af. 
the widest distribution. It was ZA hippodus, nat. size. oes from 
ceded in the Miocene pried by he John Day river, Oregon 
Fic. 9.—Skull of Castor peninsulatus Cope, nat. size. From the John Day epoch, 
Oregon. Original. 
