212 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Genus Mesosciurus gen. nov. 
Plate VIII, Figs. 1-10; Plate XIII, Figs. 11-18, Text Figs. 5, 6, 14 
(pp. 162-165). 
Type, Scourus estuans var. hoffmanni Peters. 
Size medium to above medium, much larger than in either Leptosciurus 
or Guerlinguetus; mamme, 6, instead of 8 as in Guerlinguetus; tail about 
47 to 48% of total length; character of pelage strongly variable, in correla- 
tion with the environment. 
Premolars, +. Cranial and dental characters nearly as in Guerlinguetus, 
but rostral portion of skull longer, interorbital area slightly narrower, and 
zygomata laterally more convex, giving a slightly higher ratio of zygomatic 
breadth to total length. M! and m? are nearly as in typical Sciurus; m3 
has usually two subequal prominent anterior cusps (parastyle and meso- 
style) on the outer border, but in some forms the mesostyle is decidedly 
larger than the parastyle; p‘ is usually quite similar in form and details of 
crown surface to the same tooth in Scturus. 
Geographic distribution — Northern South America, north and west of 
the Orinoco and Amazon main drainage areas, from southern Ecuador north 
to Costa Rica and Nicaragua, except southern Panama and northwestern 
Colombia. (See Map, p. 299.) 
Remarks.— Mesosciurus is the middle form among South American tree 
squirrels, in respect to both size and general details of structure, especially 
with respect to dentition. It includes three quite well marked groups, 
distinguishable by size, character of pelage, style of coloration, and geo- 
graphic distribution. The first (subgenus Mesosciurus), the hoffmanna 
group (including Sciwrus richmondi of Nicaragua), is very numerous in 
forms (6 species and 6 subspecies), and is restricted to the Andean region 
from about the northern border of Peru into Central America, and the 
northern part of Venezuela. It is usually not represented near sea level, 
although sea level forms have been developed on the coast of Ecuador 
and in Venezuela, Panama, Costa Rica, and Nicaragua. The usual alti- 
tudinal range is the subtropical and temperate zones, where it reaches 
altitudes of 8,000 to 10,000 feet. In the Central and Eastern Andes typical 
hoffmanni develops various local forms, through which, to the eastward, it 
appears almost to intergrade with the griseogena group, from which chapmant 
of northeastern Venezuela and Trinidad is not strongly different, and from 
which the insular forms of the group have been differentiated. Typical 
hof'manni presents a remarkable hiatus in distribution in northwestern 
Colombia and the southern part of Panama, where as yet no specimens have 
