FOSSIL RODENTIA 53 
ISCHYROMYID2 
Extinct. Eocene and Oligocene. 
These were Rodents with teeth more or less like Squirrels, 
but with skulls more lke those of Porcupines and Beavers. 
They are more ancient than any living rodent families, for they 
were common in the Lower and Middle Eocene, while no other 
rodents appear until the Upper Eocene. Several skulls and parts 
of skeletons of /schyromys, Paramys etc. are on exhibition. 
CASTOROIDIDA. 
Extinct. Pleistocene. 
Intermediate between beavers and hystricomorphs, but of 
very large size. Castor oluoensis equalled a black bear in size. 
A skull and jaw of this rare animal are shown in wall case No. 
8. Its remains are usually found in bogs, along with those of 
the mastodon. 
SoutH AMERICAN RODENTS. 
All the extinct and most of the living Rodents of South Amer- 
ica belong to a division more nearly related to the porcupine 
than to anything else. Some of the living ones, as the Paca and 
Capybara are of quite large size. 
A series of skulls and jaws of extinct rodents from the Miocene 
of South America is exhibited in the table-case. 
There are certain extinct European Rodent families which are 
thought to be more nearly related to the South American Rodents 
than to any others, and the remains of several of these forms are 
exhibited in the case for comparison beside them. 
LEPORIDZ. HARES AND RABBITS. 
In the White River Badlands remains of fossil Hares are very 
abundant. They are more primitive than the modern species in 
the construction of the teeth, and are placed in the genus Pale- 
olagus (‘Ancient Hare’’). In the later formations of America 
the rabbits “belong to the modern genus Lepus. 
