_Comparison._--From _Zapus princeps oregonus_, _Z. p. pacificus_ differs in being brighter in all pigmented areas; more ochraceous and less yellow laterally; dorsally more ochraceous and less black; size averaging smaller; maxillary tooth-rows shorter; auditory bullae less inflated and smaller; interorbital region averaging narrower; palatal bridge averaging shorter; incisive foramina shorter and posteriorly narrower; nasals parallel rather than narrowed posteriorly.
_Remarks._--Original describers considered both _Z. pacificus_ and _Z. alleni_ as specifically distinct from _Z. trinotatus_. Merriam (1897a:104) named _Z. pacificus_ and gave the following diagnostic characters: short rostrum and nasals; small auditory bullae; basioccipital broad between bullae. Elliot (1898:212) named _Z. alleni_ and ascribed to it the following diagnostic characters: cranium long and narrow; nasals same breadth for entire length; palate wide; pterygoid fossae wide posteriorly; auditory bullae small; basisphenoid and basioccipital wide; upper tooth-rows short. Preble (1899:27) considered _Z. alleni_ to be a subspecies of the species _Z. trinotatus_, remarking that the skulls are similar to those of _Z. trinotatus_ but smaller with much smaller bullae; in coloration the animals are lighter above and without fulvous below. Preble remarked that the skull of _Z. alleni_ differs so greatly from that of _Z. montanus_ that comparison was not required. Preble (_op. cit._:30) treated _Z. pacificus_ as a full species. Howell (1920:233) considered _Z. pacificus_ and _Z. alleni_ to be subspecies of _Z. pacificus_. Howell (_loc. cit._) pointed out size, cranial, and color similarities between the two, and remarked that _pacificus_ is clearly distinct from _Z. montanus_, its nearest geographic neighbor. Hall (1946:578) arranged _Z. alleni_ as a subspecies of _Z. princeps_, although not on grounds wholly satisfactory to him because actual intergrades between _alleni_ and neighboring races of _princeps_ were not available.
I here consider _Z. alleni_ to be synonymous with _Z. pacificus_; the latter is a subspecies of _Z. princeps_. Certain diagnostic characters, such as the shape and size of the os penis, the diameter and pigment pattern of the hair, the over-all proportions of the skull, and the size and shape of the teeth indicate that _alleni_ and _princeps_ belong to the same species, even though animals from intermediate geographic areas are not available to show actual intergradation.