17´. Length of stalk usually less than 3.6 mm., or if more than 3.6 mm. (up to 4.0 mm.) then length 1-1/2 times or more its greatest breadth 18 18. Median process attenuate distally in dorsal view, and relatively long (more than twice its own breadth), 1/5 to 3/5 the length of stalk; breadth of stalk usually 2/3 or more length of stalk.... _Microtus miurus_, p. 200 18´. Median process relatively blunt distally in dorsal view, relatively short (usually less than 1/4 length of stalk), breadth of stalk usually less than 2/3 length of stalk.... _Pitymys_, p. 208, _Pedomys_, p. 207, or _Microtus mexicanus_, p. 205 19. Distal processes small and firmly ankylosed to distal end of shaft.... _Phenacomys longicaudus_, p. 197 19´. Distal processes if present not firmly ankylosed to distal end of shaft 20 20. Dorsal concavity of base as viewed from proximal end usually deeper than ventral concavity.... _Microtus mexicanus_, p. 205 20´. Dorsal and ventral concavities of base equal in depth or ventral one the deeper 21 21. Total length of baculum more than 3.6 mm.... _Microtus californicus_, p. 205 21´. Total length of baculum less than 3.6 mm.... _Synaptomys cooperi_, p. 194 DISCUSSION Owing to shortness of lower incisors and present geographic distribution of the species, Hinton (1926:35) considered the Tribe Lemmi (lemmings) to be more primitive than the Tribe Microti (voles). The surviving lemmings are specialized in many features and therefore are considered as advanced end-products of an evolutionary radiation of a primitive microtine stock, of which all earlier stages are extinct. Hinton regarded _Dicrostonyx_ as the most primitive of the genera of lemmings on account of its more complex molar teeth (complexity was considered to be primitive), and on account of the presence of three primitive longitudinal rows of tubercles in unworn molars. The other three genera were arranged in order of increasing specialization as follows: _Synaptomys_, _Myopus_, _Lemmus_.