32 NELSON 



Extertial cha7'actcrs. — Size small — total length between 350 and 

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

 narrow and flattened. 



Crafzial 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 Guerlingiietus and much smaller 

 than in Tarniasciurus . 



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

 and S. neglige7is. In size and general style of color these squirrels 

 are very similar to members of the subgenus Guerlingiietus., 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 w^ell de- 

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

 lateral line and shape of skull separates them from Tafniasciurus. 

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

 S. 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. 



Distributio7i — Northern South America north to Costa Rica, Cen- 

 tral America. 



Extertial characters — Smallest of American squirrels — total length, 

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

 well haired; tail much shorter than body, slender and rounded. 



Cranial 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. — Microsciurus 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. pusillus 

 Desm., S. kuhli (Gray), 6". peruatius KW^w., S. 7ni?nulus Thomas, 

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



