100 
The small reddish-black tipped cusp, plainly seen on the inside of each 
molar, is very small on the first three lateral teeth. As the specimen 
becomes thoroughly dry, the line of demarcation between the lower and 
upper fur of the animal is plainly visible. 
ORDER RODENTIA. 
Incisors 3; (exceptionally 3, there being two supplemental posterior 
2) 
teeth); teeth encased in enamel, and continually reproduced from per- — 
sistent pulps, and growing in a circular direction ; canines none; molars 
attypically with ridged surfaces. Lower jaw with condyles in longitu- 
dinal furrows. Members and feet adapted for walking. Placenta dis- 
coidal deciduate. 
KEY TO FAMILIES OF RODENTIA. 
* Incisors 33 tail well developed. 
+ Hair forming a fur without spines. 
a. Tail broad, fiat and scaly; feet webbed; molars s 4; body stout and 
heavy. 0 : : A CASTORIDZ. 
aa. Tail and hind legs iene seen eae the latter adapted for leap- 
ing ; molars + ae : ; ‘ ; . A A ZAPODIDA. 
aaa, Tail usually Bae vl hairy; molars 3° 3, Or 4.7: : SCIURIDA. 
t Fur with stiffspine-like bristles; toes with long curved claws. SPALACOPODIDA. 
* * Incisors 3 ; the middle upper incisors large, grooved, the outer ones small; teeth 28; 
tail very short; ears long. : 5 : : 6 5 : 6 LEPORIDA. | 
FAMILY SCIURIDA. 
Family Characters.—Dental formula: i. }+; pm. #2 or £1; m. 33 =— 12 
or 73. Upper front premolar often deciduous; the last four grinding 
teeth of nearly equal size. Post-orbital process well developed, either 
short, pointed, and triangular, or long, slender, and much decurved. 
Palate broad and flat, extending back of last molar. Feet scansorial or 
fossorial, body generdlly elongate, tail always well haired. 
The variations in color, size, and general form, are very great, and the 
number of well defined species is very much less than was once supposed. 
Species of the family are found in all continental lands except Aus- 
tralia; they are most numerous in the Northern Hemisphere. 
Of the eight genera admitted by Mr. J. A. Allen, Scvwrus is nearly cos- 
mopolitan, Cynomys is restricted to the parks and plains of the Rocky 
Mountain plateau, Pteromys to the tropical portions of Asia, and Xerus to 
Africa. <Arctomys, Tamias, Spermophilus and Sciuropterus are found through- 
out the temperate and calles -temperate regions of the Northern Hemis- 
phere. 
Representatives of five genera are found in Ohio, as is shown in the 
accompanying key. 
