1883. ] The Extinct Rodentia of North America, 43 
THE EXTINCT RODENTIA OF NORTH AMERICA. 
BY PROFESSOR E. D. COPE, 
tae order of Rodentia appeared in the Wasatch' Eocene epoch 
in North America, and has continued to the present time in 
gradually increasing numbers. No species of the order is yet 
known from the Puerco or lowest Eocene, nor are any known 
from older formations. The Wasatch Eocene has given us but 
few species, and these are members of a single genus. In the 
Bridger epoch the number of species was larger, and they be- 
longed to several genera. The order displays a sudden expan- 
sion in the White River or Oligocene epoch. We know from 
this formation ten species of seven genera. From the John Day 
River formation we have twenty-one species of nine genera. The 
Upper Miocene Loup Fork epoch has yielded remains of nine 
species of seven genera. Four existing genera are represented 
by extinct species in the Miocene beds; two of these begin in the 
White River and two in the John Day epochs. 
The four primary divisions of the order Rodentia are thus de- 
fined, principally after Brandt and Alston: 
I. Incisor teeth 3. Fibula not articulating with the superior condyle of the calca- 
m. No intertrochlear crest of humerus. .- 
1. Mandible with the angular portion springing from the outer side of the 
my covering of the lower incisor. Fibula distinct from tibia. 
s Malar bone not supported below by a continuation of the maxillary 
i ess.” An interpterygoid fissure. .. HYSTRICOMORPHA. 
rio 
less rounded ; coronoid process high, falcate. Fibula distinct from 
tibia. No interpterygoi SUFE., cee essesceevasees ScIUROMORPHA,. 
3. Mandible with the angular portion springing from the inferior edge of the 
sheath of the inferior incisor (except Bathyerginz). Fibula coéssified 
with the tibia, Malar short, usually supported on a maxillary process. 
No interpterygoid fissure (except in Bathyergine)...... MYOMORPHA, 
II. Incisor teeth $. Fibula articulating with the condyle of the calcaneum, An 
intertrochlear crest umerus. 
4 No true alisphenoid canal ; fibula ankylosed to tibia below ; angle of man- 
fth 3 ci swe alvenins 
dible in the plane o AGOMORPHA. 
These groups, as is well known, include families and genera 
which display adaptations to various modes of life. Some are 
exclusively subterranean, others are arboreal, and some live on 
the surface of the ground. Of the latter, some are provided with 
formidable spines as a protection against enemies, while others 
depend for their safety on their speed. Of the latter character are 
1 For the positions of the American Tertiary epochs, see AMERICAN NATURALIST, 
1882, March. 
