overlap, owing to range expansion following the retreat of the ice, there is no sign of interbreeding, indicating that the populations have attained specific rank. Populations of both _Z. hudsonius_ and _Z. princeps_ occur together at Indianpoint Lake, British Columbia. Specimens taken there are readily sorted into two groups; none is intermediate. The difference in size between these species there is especially marked; _Z. p. saltator_ there is a large derivative of _Z. princeps_ and _Z. h. tenellus_ is a medium-sized _Z. hudsonius_. _Z. princeps_ minor and _Z. hudsonius intermedius_ have been taken at several neighboring localities in North Dakota. Although these geographic races are more nearly of the same size (_minor_ is a small subspecies of _princeps_ and _intermedius_ is a moderately large subspecies of _hudsonius_) they do not interbreed. Specimens of _Z. p. minor_ and _Z. h. intermedius_ have been obtained from an ecologically homogeneous area in the vicinity of Fort Totten and Devils Lake, North Dakota. Values obtained from several measurements of the skull and baculum allow for ready recognition of the two species. The populations from North Dakota are, however, not so widely divergent as are those populations from the area of contact in British Columbia. Perhaps the difference in the degree of distinction between the species at the two areas of contact is indicative of the length and completeness of geographic isolation between neighboring populations. The ranges of _Z. trinotatus_ and _Z. hudsonius_ are not at present in contact, but the two species differ more strongly than do _hudsonius_ and _princeps_ or _princeps_ and _trinotatus_. Therefore, _trinotatus_ and _hudsonius_ are here considered to be two distinct species.