1915.] Allen, Review of the South American Sciuride. 267 
(37-89); interorbital breadth, 23 (22-25); postorbital breadth, 20.5 (19-21); 
breadth of braincase, 26.5 (26-27); nasals, 20.1 & 9.2 (19-21.5 & 9-10): 
diastema, 18; maxillary toothrow, 10.5 (10-11). 
Specumens examined, 17.— Venezuela: La Union, 8 (Br. Mus. 2, type 
and paratype; Am. Mus. 6); El Llagual, 2 (Am. Mus.); Ciudad Bolivar, 1 
(Am. Mus.); Rio Mocho, 1 (Am. Mus.); Suapure, 5 (Am. Mus.). - 
Remarks.— The geographic relationship of Hadrosciurus flammifer to 
the Urosciurus group is little known. U. duida occurs on the upper reaches 
of the main Orinoco, the U. agniventris group on the Rio Negro and in the 
Cundinamarca and Caqueta districts of Colombia, to which flammzfer 
is much more nearly related than to U. tracolor or U. duada. 
Genus Urosciurus gen. nov. 
Text Figs. 10, 16 (pp. 164-165); Plates X and XI; Plate XIV, Figs. 1-6. 
Type, Sciurus tricolor Poeppig. 
Size large; tail long, broad, and bushy, the vertebrae about 50 to 52 % 
of the total length; mamme, 8; pelage usually thin and short, often very 
thin on the ventral surface. 
Premolars, 7. Skull long and narrow, of medium depth (about 36 % of 
total length at m?), and only moderately convex, the mid-dorsal outline 
nearly straight; length of nasals 31 to 33 % of total skull length; zygomata 
evenly convergent anteriorly, the breadth of the skull at m' being much less 
than at the posterior border of the zygomatic fosse (about 50 % instead of 
56 % of total skull length); malar weak, narrow, superior process slightly 
developed - dentition weak; molars with the cusps on outer border small 
and low, the intervening cusplets nearly suppressed; lower incisors long, in 
correlation with the long rostrum. 
Geographic distribution.— Drainage basins of the upper Orinoco and 
middle and upper Amazon. (See map, p. 300.) 
Remarks.— The special features of Urosciurus are the long, broad tail, 
which appears to reach its maximum development in U. duida (q. v.); the 
eradually anteriorly converging outlines of the skull, from the posterior 
border of the zygomatic fossee to the end of the long narrow rostrum, the 
depressed dorsal outline, the general narrowness of the skull in proportion 
to its length, and the weak, simple dentition. The genus includes U. 
tricolor, pyrrhonotus, igniventris, duida, and langsdorffii, with their respective 
subspecies. The nasals vary in their posterior extension, reaching further 
back in tricolor than in the igniventris and ‘duida groups. U. langsdorffi 
is aberrant, but seems better placed here than in any of the other generic 
groups here recognized, and not sufficiently differentiated to warrant 
