SQUIRRELS OF MEXICO AND CENTRAL AMERICA 27 



America there seems to be a curious correlation between the increased 

 thickness of the rostrum and the increased coarseness of the pelage, 

 the extreme of which is reached in S. t}io7nasi. 



HESPEROSCIURUS^ subgen. nov. (p1. I, fig. 5). 



Type Sciiirus griseus Ord, from the Dalles of the Columbia. 



Distribution. — Extreme southwestern Washington, western Ore- 

 gon, and most of California, to northern Lower California, Mexico. 

 Transition zone and border of Upper Sonoran. 



External characters. — Size very large — total length about 560 mm. ; 

 tail a little shorter than head and body, very broad and bushy. 



Cranial characters. — Premolars f ; molar series, including small 

 premolar, unusually heavy ; skull large and long, with long and deep 

 rostrum ; nasals long, expanded anteriorly and much narrowed poste- 

 riorly ; brain case depressed and broadened across parietal region ; pos- 

 torbital process of malar strongly developed and usually ending in a 

 point. Skull most like that of Neosciurns, especially in proportions 

 of rostrum and interorbital area, but decidedly more depressed and 

 broadened posteriorly ; the zygomatic process of squamosal thrown out 

 horizontally, and the zygomatic arch inclined less obliquely upward. 

 General notes. — Hesperosciurus is a small group containing only S. 

 griseus Ord and its subspecies nigripes Bryant. It is limited to the 

 Transition and upper part of Upper Sonoran zones of the Pacific Coast 

 States and has its nearest relative in the Gray Squirrels {Neoscizcrtis) 

 of the eastern United States. Like the latter the color is uniform 

 gray above and white below, the group distinctions resting mainly on 

 cranial characters. 



Subgenus NEOSCIURUS Trouessart. 



Neosciurtis Trouessart, Le Naturaliste, II, No. 37, Oct. 1880, p. 



292; Cat. Mamm., Rodentia, pp. 76-77, 1880 (part); Merriam, 



Proc. Biol. Soc. Washington, VII, p. 27, 1892 (part). 

 J/a<:ro.v;/5 Trouessart, Catalogus Mammalium, nov. ed., II, p. 421, 



1897 (part). 



Type Sciurus carolinensis Gmelin, from Carohna. 



Distribution.— Y.A^iern half of United States, not reaching Mexican 

 border. Lower and Upper Austral and Transition zones. 



External characters.— Form rather slender; tail bushy; upper- 

 parts gray or grayish brown; underparts white. Externally much 

 like certain species of Ara:osciurus but with different tooth formula. 



1 From ea-ivepog, western ; -f Sciurus. 



