32 NELSON 



External characters. — Size small — total length between 350 and 

 400 mm.; ears long, thinly haired; tail slightly shorter than body, 

 narrow and flattened. 



Cranial characters. — Premolars |^, well developed; skull rather 

 long and slender; rostrum broad and deep at base; nasals about equal 

 to interorbital breadth ; anterior end of zygomatic arch narrow and 

 tapering gradually to junction with premaxilla ; braincase rounded 

 and slightly arched on posterior half of frontals ; audital bullae propor- 

 tionately small, smaller than in Guerlinguetus and much smaller 

 than in Tatniasciurus. 



General notes. — Baiosciurus contains but two species, S. deppel 

 and S. 7ieglige7is. In size and general style of color these squirrels 

 are very similar to members of the subgenus Gue7-li7iguetus., but are 

 distinguished by the presence of an extra premolar and other skull 

 characters. Their size, shape of tail, and form of skull distinguish 

 them from Microsciurus ; and the slender tail, presence of a well de- 

 developed small premolar, lack of ear tufts, absence of the black 

 lateral line and shape of skull separates them from Tamiascttirus. 

 The group is purely tropical, S. deppei belonging to the Humid and 

 ■5". negligens to the Arid Tropical zones. 



Subgenus MICROSCIURUS Allen (p1. I, fig. 6). 



Microsciurus Allen, Bull. Am. Mus. Nat. Hist., VII, p. 333, 1895 ; 



Trouessart, Catalogus Mammalium, nov. ed., II, p. 429, 1897. 

 Type Sciurus alfari Allen, from Jimenez, Costa Rica. 



Distribution — Northern South America north to Costa Rica, Cen- 

 tral America. 



External characters — Smallest of American squirrels — total length, 

 in nearly or quite all the species, under 300 mm. Ears short, rounded, 

 w^ell haired; tail much shorter than body, slender and rounded. 



Cratiial characters — Premolars y. Skull short, broad and highly 

 arched over braincase : rostrum short, broad and deep at base ; nasals 

 narrow and shorter than interorbital breadth ; upper end of premax- 

 illae very heavy; malar broad and expanded vertically; postpalatal 

 notch only a trifle posterior to last molar. 



General notes.— JMicrosciurus is a tropical American subgenus 

 with numerous species distributed over a large part of northern South 

 America and may be considered intrusive in Central America, S. al- 

 fari being the only species known north of Panama. S. piisillus 

 Desm., S. kuhli (Gray), S. peruajius Allen, S. rtiimuhis Thomas, 

 and other South American species belong here. Until Dr. Allen de- 



