_Range._--Southern and western Marin County, California. Zonal range: Upper Sonoran areas that are moist yet safe from continuous inundation.
_Description._--Size small; back dark ochraceous, usually overlaid with black hairs forming broad dorsal band; side lighter than back with admixture of black hairs; lateral line distinct, usually bright, near Ochraceous-Buff; under parts strongly suffused with ochraceous; tail bicolored, white to yellowish-white below and dark brown above; feet grayish-white above; ears dark, edged with yellowish-white or tan; skull small; zygomata narrow; braincase narrow; maxillary tooth-rows short; interorbital region narrow; incisive foramina short; palatal bridge relatively long; mastoid region relatively broad; occipitonasal length short.
_Comparison._--From _Zapus trinotatus eureka_, _Z. t. orarius_ differs in: Size smaller; color, dorsally and laterally, brighter, more ochraceous; skull averaging smaller in all measurements taken except length of palatal bridge, where it averages longer; auditory bullae smaller, less inflated; pterygoid fossae narrower.
_Remarks._--Preble (1899:30) named this jumping mouse as a full species. Included in the specimens examined were animals from Eureka and Mad River, Humboldt County, California. Howell (1920:231) retained _Z. orarius_ as a full species but restricted its range to Marin County, California, and referred material from northern California, including the animals from Eureka and Mad River, to a new subspecies (_eureka_) of the species _Z. trinotatus_. Howell (_loc. cit._) suggested that _Z. orarius_ had its closest affinity with _Z. t. eureka_ but remarked that intergrading material was not available. Hooper (1944:68) arranged _Z. orarius_ as a subspecies of _Z. trinotatus_ and suggested that intergrades could be expected from geographically intermediate areas, for example, northern Sonoma County, California.
Although animals from intermediate geographic areas still are not available to show actual intergradation, I concur with Hooper (_loc. cit._) and arrange _Z. orarius_ as a subspecies of _Z. trinotatus_. The close relationship of _Z. orarius_ to _Z. trinotatus_ is evident; certain diagnostic characters, held in common, are the shape and size of the os penis, the diameter and pigment pattern of the hair, and the general configuration of the skull.