Type.—Obtained at Coeur d'Alene, Kootenai County, Idaho, by C. P. Streator on June 1, 1891.
Measurements.—Six males and 3 females from northeastern Washington average: total length 234; length of tail 109; hind foot 31.6; ear 18.
Distribution.—Northeastern Washington, reported from Pend Oreille, Stevens and Ferry counties by [Howell] (1929: 98).
Description.—The red-tailed chipmunk closely resembles Tamias amoenus. From T. a. luteiventris and T. a. canicaudus it differs principally in larger size, wider brain case and especially in its pure white underparts. From T. a. affinis it differs in richer coloration, especially the brownish rather than gray rump. The differences separating it from amoenus are slight, and only adult specimens can be identified in the field.
According to [Howell] (1929: 81) this species occurs in northern Idaho, western Montana, northeastern Washington, southeastern British Columbia and extreme southwestern Alberta. Two subspecies are recognized, only one of which occurs in Washington.
A number of large, white-bellied chipmunks have been taken in northeastern Washington that answer well to the description of ruficaudus. Also, there are a number of specimens that I cannot definitely identify as either amoenus or ruficaudus. Some buff-bellied chipmunks from Idaho are as large, and possess brain cases as wide, as specimens from Washington unhesitatingly called ruficaudus, while some white-bellied individuals match amoenus luteiventris in all other characters. When all the specimens available from Pend Oreille, Stevens and Ferry counties are separated into amoenus and ruficaudus and the skulls are examined, it is noticeable that all the ruficaudus are old, fully adult animals and that most of the amoenus are younger, showing less wear on the teeth. Perhaps the buffy underparts are lost with increasing age.
This leads to the suspicion that ruficaudus, as applied to chipmunks in Washington, is a synonym of amoenus. Until considerable material is collected in northeastern Washington, showing individual and age variation, it seems best to retain the name ruficaudus.
Most of the specimens referable to ruficaudus were taken in talus slides high in the Pend Oreille Mountains. A smaller series was collected at a lower altitude in open pine forests near Pend Oreille Lakes, Stevens County. I could detect no difference between amoenus and ruficaudus in habitat or habits.