86 NELSON 



white ring around eve ; rest of sides of head finely grizzled gray, 

 paler than back ; ears dingy gray and thinly haired, sometimes 

 with traces of grizzled gray and black winter tufts ; underparts white ; 

 tail at base all around like rump ; rest of tail above black, heavily 

 washed with white ; below uniformly grizzled gray, with narrow indis- 

 tinct black border, a well marked subterminal black area, and broad 

 white edging. Hairs on back black with two to three rings of grayish 

 white, the rings often becoming rufous on middle of back. 



Variation. — In some specimens the upperparts are nearly uniform 

 dingy gray, the chestnut dorsal area being indicated merely by a rusty 

 suffusion along the lumbar region. In those with a distinct chestnut 

 dorsal area, numerous rufous ringed hairs are scattered over the back 

 and flanks, giving the gray a slight brownish tinge. The color of the 

 hind feet is very variable and is scarcely alike in any two specimens, 

 but is never pure white as in aberti. One of the most striking and 

 constant characters of this species is the entire absence of white on 

 the underside of the tail. 



JMeastirements. — Average of five adults from vicinity of type lo- 

 cality : total length 4S6.4; tail vertebrse 235.4; hind foot 71.2. Five 

 adult topotypes of 6". ai^er// average : total length 510 ; tail vertebra; 

 229.8; hind foot 75.6. 



Cranial characters. — Premolars y* Skull (pi. I, fig. 2) generally 

 similar to that of S. aberti but somewhat smaller, with notably 

 smaller molars, and shorter and more slender first premolar ; audital 

 bullae relatively larger; braincase a little less draw^n out posteriorly. 

 Five adult skulls from vicinity of type locality average as follows: 

 basal length 50.1 ; palatal length 27.3; interorbital breadth 19.8 zy- 

 gomatic breadth 34.5; length of upper molar series 10.8. Five adult 

 skulls of 6^. aberti from type locality average: basal length 52; pa- 

 latal length 27.8; interorbital breadth 19.8; zygomatic breadth 35.1 ; 

 length of upper molar series 11.2. 



General notes. — The characters which separate this squirrel fi'om 

 S. aberti are so constant in the series before me that it seems best to 

 consider it a distinct species. Each species has a well defined range, 

 there being a broad gap in northern Chihuahua and southern Arizona 

 in which neither occurs. 



Specimens exaiuined. — Thirty-two: from El Salto (near Ciudad 

 ranch), Durango; Sierra Madre and near Guadalupe y Calvo, Chi- 

 huahua (in southwestern corner of State). 



