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 |; (exceptionally f, there being two supplemental posterior 

 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 op Eodentia. 



* Incisors 5 j tail well developed. 



t Hair forming a far without spines. 



a. Tail broad, flat and scaly; feat webbed; molars ^if ; body stout; and 



heavy. CASTORiDiE. 



a a. Tail and hind legs excessively elongated ; the latter adapted for l»ep- 



ing; molars fl|-. Zapodidje. 



a a a. Tail usually long and hairy ; molars jif , or \z^. . SciURiD^. 



+ Fur with stiff spine like bristles; toes with long curved claws. SPALACOPODrD^. 



* Incisors ^ ; the middle upper incisors large, grooved, the outer ones small; teeth 28; 



tail very short ; ears long. LBPORlDiE. 



FAMILY SCIURIDiE. 



Family Characters. — Dental formula : i. yiy; pm. yll or \'.\; m. f:|- = -j-f 

 or \-g. Upper front premolar often deciduous; the last four grinding 

 teeth c£ 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 las* molar. Feet scansorial or 

 fossorial, body generally 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, Sciurus is nearly cos- 

 mopolitan, Cynomys is restricted to the parks and plains of the Rocky 

 Mountain pkteau, Reromys to the tropical portions of Asia, and Xerus to 

 Africa. Arctomys, Tamias, Spermophilus and Sciuropterus are found through- 

 out the temperate and cold-temperate regions of the Northern Hemis- 

 phere. 



Representatives of five genera are found in Ohio, as is shown in the 

 accompanying key. 



