Descriptions of East Asiatic Mammals 211 
Moreover, directly behind this upper pair of clipping teeth is a 
pair of much smaller teeth, the like of which is found in no 
true rodent. Numerous other important differences contribute 
to the view that Hares and their allies are at best remotely re- 
lated to Squirrels, Rats, and Porcupines. 
Even leaving aside the Hares, the three groups of True Ro- 
dents are divided from one another by gulfs of structural differ- 
ence. Many millions of years ago the Sciuromorpha or "squirrel- 
shaped," the Hystricomorpha or "porcupine-shaped," and the 
Myomorpha or "mouse-shaped" rodents were already com- 
pletely distinct. There are numbers of seeming exceptions to 
this classification in each of the suborders, due to the use of the 
word "shaped" ; there exist, for example, mouse-shaped Sciuro- 
morpha and Hystricomorpha, but the detailed anatomy of such 
animals proves their true affinities. 
squirrel-like rodents and relatives 
(suborder sciuromorpha) 
This suborder comprises the superfamily Sciuroidea, Squir- 
rels and Beavers; the American Pocket Gophers and relatives, 
superfamily Geomyoidea; and the American Mountain Beavers, 
Aplodontoidea. 
superfamily aplodontoidea 
The Sewellels or Mountain Beavers of the American Pa- 
cific coast are no longer found in Asia. That they existed there 
formerly is proved by discovery in Mongolia of the fossil form, 
Aplodontia asiatica. 
THE SQUIRRELS AND BEAVERS 
(superfamily SCIUROIDEA) 
The idea that Squirrels, Woodchucks, and Beavers are built 
on the same broad structural plan and are more closely related 
