Evotomys nivarius Bailey, Proc. Biol. Soc. Washington, 11:136, May 13, type from northwest slope of Mount Ellinor, 4000 ft., Olympic Mts., Mason County, Washington.

The red-backed mouse of the Olympic Peninsula was originally accorded specific rank. Currently it stands in the literature as a subspecies of the wide-spread species Clethrionomys gapperi because Dalquest (Univ. Kansas Publ. Mus. Nat. Hist., 2:343, April 9, 1948) used the name-combination Clethrionomys gapperi nivarius. Taylor and Shaw had earlier (Occas. Papers Charles R. Conner Mus., 2:23, 1929) indicated the same status by using the name Evotomys gapperi nivarius. Davis (The Recent Mammals of Idaho, The Caxton Printers, Caldwell, Idaho, p. 306, April 5, 1939), however, indicated that the affinities of nivarius were with the californicus [= occidentalis] group, although he treated nivarius as a distinct species. We have examined two adult females (K. U. Nos. 10707 and 10708) of nivarius from Reflection Lake, 3800 ft., Jefferson County, Washington, and on the basis of their thick, instead of thin, pterygoid processes concur with Davis that the affinities of nivarius are with the named kinds of Clethrionomys now arranged as subspecies of Clethrionomys occidentalis, rather than with the kinds now arranged as subspecies of Clethrionomys gapperi. Although we are aware that Dalquest (op. cit.:101-102) did not find actual intergradation between nivarius and Clethrionomys occidentalis occidentalis—a ten-mile gap separated their ranges—we prefer to use the name combination Clethrionomys occidentalis nivarius. In doing so we recognize that intergradation ultimately may be found between the two species C. occidentalis and C. gapperi; in that event the name gapperi will apply as the name of the species because it has priority over occidentalis.

The following named kinds of Clethrionomys are considered to be subspecies of Clethrionomys occidentalis:

Clethrionomys occidentals occidentalis (Merriam).

Clethrionomys occidentalis californicus (Merriam).

Clethrionomys occidentalis caurinus (Bailey).

Clethrionomys occidentalis mazama (Merriam).

Clethrionomys occidentalis nivarius (Bailey).

Clethrionomys occidentalis obscurus (Merriam).

The name rufescens, as applied by R. W. Smith (Amer. Midland Nat., 24:233, July, 1940) to the red-backed mouse of Nova Scotia, seems to be unavailable under the rules of the International Code of Zoological Nomenclature, since it is a homonym of Arvicola rufescens de Selys Longchamps, 1836, which in turn is a synonym of Clethrionomys glareolus glareolus Schreber, 1780 (Ellerman and Morrison-Scott, Checklist of Palaearctic and Indian Mammals, 1758 to 1946, p. 663, November 19, 1951).

When Swarth (loc. cit.) named the red-backed mouse of the mainland of southern Alaska as a new subspecies, he characterized it as "Size rather large. Differs from E. [= Clethrionomys] wrangeli, nearest it geographically, in cranial characters and in much longer tail; from E. caurinus, the species to the southward in British Columbia, in larger size and longer tail." He remarked (loc. cit.): "I had supposed that the red-backed mouse occurring on the mainland coast of this region would prove to be E. wrangeli, but the latter appears to be purely an insular species. I have had no specimens of that race for comparison, but the Evotomys secured differ so widely from it in all the essential peculiarities of the species as given in the published descriptions that there seems little doubt of their belonging to a different species. Wrangeli has a short tail, less than twice as long as the hind foot—in adults of phaeus the tail is invariably more than twice the length of the foot, frequently more than a third of the entire length of the animal."

The external and cranial measurements of two subadults in the United States National Museum (No. 217413 from Quadra Lake and No. 217415 from Marten Arm, Boca de Quadra, taken in mid-February) and three old adults from Fort [= Port] Simpson, British Columbia (Nos. 90263-90264, 90272 USBS), are almost the same as those given by Swarth in the original description of Clethrionomys phaeus.

In cranial measurements, as well as in the structure of the palate and last upper molar, C. phaeus agrees with the gapperi group (to which it has been assigned by Davis, The Recent Mammals of Idaho, The Caxton Printers, p. 306, April 5, 1939, and by Orr, Jour. Mamm., 26:69, February 12, 1945) and differs from Clethrionomys occidentalis caurinus (which was assigned above to the occidentalis group, formerly the californicus group).

Since the measurements of specimens examined by us, as well as those recorded by Swarth (op. cit.), fall within the range of those of the species Clethrionomys gapperi, and since the differences between phaeus and C. g. saturatus are of the kind and degree that separate subspecies in C.