188 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Genus Microsciurus Allen. 
Plate VII, Figs. 4-6; Plate XIII, Figs. 3,4; Text Fig. 1 (p. 162). 
Microscvurus ALLEN, Bull. Amer. Mus. Nat. Hist., VII, p. 332, Nov. 8, 1895 
(subgenus of Sciwrus).— Netson, Proc. Washington Acad. Sci., I, p. 32, pl. i, fig. 6, 
pl. u, fig. 2, May, 1899 (subgenus of Sciurus)— GotpMan, Smithson. Mise. Coll., 
LVI, No. 36, p. 4, Feb. 12, 1912 (genus).— Mitimr, Bull. 79, U. S. Nat. Mus., p. 
338, 1912 (genus).— ALLEN, Bull. Amer. Mus. Nat. Hist., XX XIII, pp. 145-165 
(genus; monographic review). 
Type, Sccurus (Microsciurus) alfari Allen. 
Smallest of the American tree squirrels, except Sciurillus. Total length 
about 240 to 260 mm.; tail short and narrow, tail vertebree about 40% 
of the total length (7. ¢., tip of nose to end of tail vertebree). Mamme, 6. 
Premolars, #. Skull short, broad, and deep; greatest width near the 
front border of the zygomatic fosse (at m!), equal to about 60% of total 
length of skull; dorsal outline highly arched, the highest point at the post- 
orbital processes; nasals short and broad, their length about 28% of the 
total length of the skull, and about 76% of the interorbital breadth; breadth 
of braincase 50% of total skull length. Orbital fossa circular, when seen 
from above nearly closed posteriorly, the open space behind the postorbital 
process very small instead of forming one third or more of the whole space, 
as in other tree squirrels; zygomata converging posteriorly instead of ante- 
riorly; malar broad, with a deep depression in the superior border just 
behind the middle, as in Nannosciurus. Upper molars nearly normal in 
form and position, parastyle, mesostyle, and metastyle strongly and about 
equally developed in p‘, m! and m2, without the intermediate cusplets 
usually present; transverse ridges on crown strongly developed, but di- 
rected obliquely internoposteriorly instead of transversely or anteriorly; 
p* is nearly as large as m? and similar to it in outline; p® is a well developed 
column reaching the level of the other teeth, with often a bicuspid func- 
tional crown. 
Geographic distribution— The western Andean region of Colombia, 
south to the southern border of Peru, north to central Costa Rica. 
Represented by 17 described species and subspecies. (See Map, p. 298.) 
Remarks.— The distinctive external features of the genus are small size, 
a short narrow tail, and usually prominent postauricular patches of long 
soft whitish or buffy hair. The shape of the skull is widely different from 
that of ordinary sciurids, the dorsal outline being much more convex, the 
braincase greatly expanded and deep, the greatest expansion of the zygo- 
matic arches near the front border instead of at the middle or posterior to 
the middle. The well developed and functional p® is rarely absent. 
