174 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Genus Hesperosciurus. 
Plate III, Figs. 4-6; Plate VI, Figs. 14, 15. 
Hesperosciurus Nelson, 1899 (subgenus of Sciwrus). Type, by original 
designation and monotypy, Scvurus griseus Ord. 
Size very large, tail very long and full (about 50% of total length); 
coloration gray above, white below, without special markings. Mamma, 8. 
Premolars, #, p? heavily developed; m* with a single strongly developed 
conical cusp, with the accessory cusplets nearly suppressed; dentition other- 
wise as in Neosciwrus and Otosciurus. Skull massive, heavily built, but 
contour and proportions nearly as in Neosciurus and Otoscturus; malar 
heavier, process of superior border better developed. 
Range, Pacific coast of the United States, from southwestern Washing- 
ton to northern border of Lower California. 
Species: Hesperosciurus griseus, with several subspecies. 
The practically monotypic genera Neosciurus, Otosciurus, and Hespero- 
scrurus are closely related genetically but widely separated geographically; 
they have become strongly differentiated in external features, much less 
so in cranial.characters and in dentition. Otosciurus and Hesperosciurus 
could very well stand as subgenera of Neosciurus. 
Genus Echinosciurus. 
Plate II, Figs. 1-8; Plate VI, Figs. 8-11. 
Echinosciurus Trouessart, 1880 (subgenus of Sciwrus). Type, by original 
designation, Sciurus hypopyrrhus Wagler (= Sciurus aureogaster hypopyr- 
rhus Nelson). 
Size large; tail long, about 50% of the total length (49 to 51 in different 
species); coloration variable; texture of pelage soft or hispid, according to 
the environment. Mamme, 8. 
Premolars, ?, p? usually small, slender ; molar dentition not distinctive. 
Skull broad, dorsal outline flattened or slightly swollen at frontal region; 
rostrum short, length of nasals about 95% of interorbital breadth (in the 
type species reaching 100% in some specimens). 
Range, southern Mexico, south to northern Panama. 
Species numerous (about 16, some of them with numerous subspecies), 
variable in cranial details. : 
The type species is a middle form in respect to the shape of the skull. 
The E. poliopus, E. socialis and E. sinaloensis groups have a longer ros- 
