56 NELSON 



nearly uniform blackish with yellowish gray or brown showing through 

 and thin edging of dingy white, but sometimes wnth median area very 

 dark grizzledyellowish gray or yellowish brown, washed with black and 

 narrowly edged with dingy white. Hairs on entire back black with 

 narrow subterminal and broader basal or sub-basal rings of smoky buff 

 or brown. 



Variation. — No melanistic specimens have been seen from the type 

 locality, but two individuals from Tenango del Valle in the Bangs 

 collection are nearly uniform black, having only a thin grizzling of 

 dingy gray along the sides, and one has a pale wash of reddish brown 

 on breast and belly. Another specimen in the same collection from 

 Tenango differs in having a nuchal patch of dark grizzled reddish 

 brown, contrasting with rest of back; feet pale buffy brown, toes 

 dingy whitish; rest of back normal, but entire underparts dull 

 rufous, becoming darker posteriorly. One specimen from Ajusco also 

 has the underparts similarly colored. 



Measurements. — Average of five adults from type locality : total 

 length, 521.6; tail vertebrae, 262.6; hind foot, 67.6. 



Cranial characters. — Premolars \. Skull like that of S. atireo- 

 gaster. Five adult skulls from the type locality measure : basal 

 length 50.7; palatal length 26.7; interorbital breadth 19.4: zygomatic 

 breadth 34.9; length of upper molar series 11.9. 



GeJteral notes. — Sciurns nelsoni is one of the most distinct of the 

 Mexican squirrels and has a very limited distribution. It has a single 

 subspecies which lives on the slopes of Mts. Popocatepetl and Iztac- 

 cihuatl, and serves as a good illustration of the remarkable readiness 

 with which the squirrels of tropical North America vary with climatic 

 conditions. The area inhabited by S. nelsoni and S. nelsoni hirtus 

 are in the same life zone and not more than 75 miles apart. S. nelsoni 

 is much the darker of the two and its coloration is correlated with the 

 somewhat greater rainfall of the area it inhabits. 



Specimens examined. — Eighteen : from Huitzilac, Morelos ; Salazar 

 and Ajusco, Federal District ; and Tenango del Valle, State of Mexico, 

 Mexico. 



SCIURUS NELSONI HIRTUS Nelson. 



Mount Popocatepetl Squirrel. 



Sciuriis nelsoni hirtus Nelson, Proc. Biol. Soc. Washington, xii, pp. 153- 

 154, June 3, 1898. 



7ype locality. — Tochimilco, Puebla, Mexico. Type no. 55325 

 U. S. National Museum, Biological Survey Collection. 



