172 Bulletin American Museum of Natural History. [Vol. XXXIV, 
species, and may become greatly reduced in size or even obsolete. In respect 
to external features, the mamme number either 6 or 8; the tail varies in 
length in accordance with the habits of the species. Taking the total length 
from the tip of the nose to the end of the tail vertebre (not to the end of the 
hairs) as 100, the ratio of the length of the caudal vertebre to the total 
length is found to vary only from about 40 to 52; in other words, from 
considerably less to slightly more than the head and body length. The 
limbs and feet, under the control of scansorial adaptation, are short and 
strong, with the hind limbs never lengthened. The rostrum and incisors 
are modeled for strength and efficiency in extracting the nutrient kernels 
from nuts and husks. Consequently although tree squirrels constitute a 
numerous group their arboreal adaptation restricts the range of their 
structural modifications to narrow limits. It is hence apparently desirable 
to emphasize such features of differentiation as may be available in order to 
indicate the genetic interrelationships of the considerable number of super- 
specific natural groups so long hidden under the old Linnean name Sciurus. 
North American genera. 
My present views on the classification of North American tree squirrels 
are here presented, with diagnoses of the generic and subgeneric groups, 
and illustrations of cranial characters and dentition. 
Genus Tamiasciurus. 
Plate I1, Figs. 10-12;]Plate VI, Figs. 2, 3. 
Tamiasciurus 'Trouessart, 1880 (subgenus of Sciurus). Type, by original 
designation, Sciurus hudsonicus Erxleben. 
Size small (smallest of North American arboreal squirrels); tail short 
and narrow, about 40% of total length); coloration distinctive; a conspicu- 
ous black lateral line in summer pelage. Mamme, 8. 
Premolars, ?, p’ vestigial; dentition otherwise similar to that of restricted 
Sciurus. Skull rather narrow for the length, moderately convex, the high- 
est point at the fronto-parietal suture; nasals short, narrow, posterior border 
obtusely V-shaped; length of nasals to total length, about 287,)(85°7 in 
S. vulgaris); interorbital breadth to total length, 30% (37% in S. vulgaris); 
zygomatic breadth to total length, 56% (68% in S. vulgaris). 
Species: Tamiasciurus hudsonicus, T. douglasii, T. fremonti, each with 
numerous subspecies. 
1¥For explanation of Plates I-VI, see pp. 179, 180. 
