When Dalquest (op. cit.:344) arranged occidentalis and californicus as subspecies of the same species, he used the name combination Clethrionomys californicus occidentalis because he ignored, or was unaware of, the page priority of occidentalis over californicus. We regard the anterior position of occidentalis as nomenclatural priority and therefore employ occidentalis rather than californicus as the specific name.
Differences between the gapperi group and the occidentalis group include: postpalatal bridge (complete in both groups) truncate posteriorly in the gapperi group and with a median, posteriorly directed, spine in the occidentalis group (this character is not evident in all specimens; some gapperi have a spine, and some occidentalis have the spine much reduced); dentition of the occidentalis group is heavier; enamel pattern of M3 and m1 in occidentalis more simplified—the number of salient and re-entrant angles tends to be reduced in adults of the occidentalis group.
An examination of specimens of caurinus (British Columbia: Mt. Seymour, 2 KU; Lund, Malaspina Inlet, 2 USBS; and Inverness, mouth Skeena River, 1 USBS), reveals that, in the presence of the median postpalatal spine and in the characters of the molars, caurinus agrees with the occidentalis group.
Clethrionomys occidentalis nivarius (Bailey)
1897. 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).
1890. Evotomys occidentalis Merriam, N. Amer. Fauna, 4:25, October 8, type from Aberdeen, Chehalis County, Washington.
1894. Evotomys pygmaeus Rhoads, Proc. Acad. Nat. Sci. Philadelphia, p. 284, October 23, type from mouth of Nisqually River, Pierce County, Washington.