1915.] Allen, Review of the South American Sciuride. 169 
species from the same locality. The dorsal contour of the skull varies with 
the age of the individual, and also in those of the same age, as does the 
relative development of the rostrum, the form of the nasals, and the relative 
interorbital breadth. These, however, are less disturbing than the vari- 
ability in the form and in the details of the crown pattern of p* and m3, and 
to a less extent in mt and m*. Single skulls of various species were taken at 
random for photographing and for detailed study. In several instances 
what were thought to be important characters were discovered, and later 
when, to make sure of their diagnostic value, they were checked up by com- 
parison with a series, it was found that their importance vanished, as the 
differences proved to be not constant. 
GENERA AND SUBGENERA OF AMERICAN TREE SQUIRRELS. 
The American tree squirrels are separable into a number of fairly well 
circumscribed, and therefore natural, groups, not all, of course, of the 
same degree of differentiation. None are strictly congeneric with Sciurus, 
sens. stric., typified by Sciurus vulgaris Linné of Europe and Northern Asia. 
In 1880 Trouessart, in his revision of the genus Seiwrus,! divided the Ameri- 
can species into five subgenera, as follows: (1) Neosciurus, (2) Parasciurus, 
(3) Macroxus (= Guerlinguetus Gray, 1821), (4) Echinosciurus, (5) Tamia- 
-sciurus. Another, Microsciurus Allen, was added in 1895. In 1899 all of 
them were accepted by Nelson (Proc. Washington Acad. Nat. Sci., I, pp. 
15-106), who proposed four more, namely: (1) Hesperosciurus, (2) Oto- 
sciurus, (3) Areosciurus, (4) Batosciurus. Another, Syntheosciurus, was 
added as a full genus by Bangs in 1902, making eleven generic and sub- 
generic groups for the American tree squirrels found north of Panama. In 
June, 1914, Thomas added Sciurillus, as a full genus, for the pygmy squir- 
rels of Guiana, which he referred to the subfamily Nannosciurine, previously 
known only from West Africa and the Malay Archipelago. In the present 
paper seven additional subdivisions are recognized for groups occurring 
only in South America, namely, Notosciurus, Leptosciurus, Mesosciurus, 
Histriosciurus (as a subgenus of Mesosciurus), Hadrosciurus, Uroscwurus, 
and Simosciurus. 
In 1912, G. S. Miller, Jr., in his ‘ List of North American Land Mammals’ 
(Bulletin 79, U. S. Nat. Mus.), recognized 38 species and 58 additional sub- 
species (96 forms) of tree squirrels as occurring north of Panama, referred by 
him to three genera and three additional subgenera, four of the ten sub- 
1 Revision du genre Ecureuil (Sciurus), La Naturaliste, No. 37, pp. 290-293, Oct. 1, 1880. 
