24 NELSON 



in thirty percent of the specimens examined. In the skulls of 

 Tamiasciiirus examined by me the percentage of absences is 

 much smaller than that given by Dr. Allen, but the tooth was fre- 

 quently so minute that it could be seen only by the aid of a lens. 

 It is smallest, most variable and most frequently absent in the 

 eastern S. htidsoniciis and its subspecies. In the other groups 

 of North American squirrels, this tooth, when present, is a well- 

 developed peg, reaching nearly or quite as high as the 2d pre- 

 molar, and often functional, the crown wearing down with the 

 adjacent teeth. Skulls from Mexico show conclusively that in 

 the species of that region, at least, the small premolar is a char- 

 acter of the adult. It is not present with the milk premolar in 

 immature skulls, but appears coincidently with the permanent 

 premolar and is equally persistent. Since it is persistent in cer- 

 tain groups and always absent in others, it evidently possesses 

 considerable taxonomic value. 



The subgenera recognized in the present paper occupy 

 clearly defined geographic areas and, without exception, the 

 ranges of the most closely related groups are separated by 

 a distinct gap. A curious equality is found in the number of 

 subgenera belonging respectively to Tropical and Temperate 

 North America. Five of the ten subgenera belong essentially to 

 Mexico and Central America, only one having a representative 

 within the United States. The remaining five belong to Tem- 

 perate North America, although four of them have each a single 

 representative in extreme northern Mexico. The distribution of 

 the subgenera may be tabulated roughly as follows : 



MEXICO AND CENTRAL AMERICA. UNITED STATES. 



Baiosciurus subgen. nov. Tamiasciiirus Trouess. 



EcJiinosciurus Trouess. JSfeosciurus Trouess. 



ArcEosciurus subgen. nov. Parasciurus Trouess. 



Not represented. Otosciurus subgen. nov. 



Not represented. Hesperosciurus subgen. nov. 



Giierlingtietus Gray. Not represented. 



Microsciurus Allen. Not represented. 



The extra premolar is present in all of these groups except 

 ArcBoscitirus, Parasciurus, and Guerlingiietus. Gucrlinguetiis 



