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
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_.
If the baculum tended to retain its primitive character while specializations in the external anatomy developed, and if the above arrangement is correct the most primitive bacula would be found in _Dicrostonyx_ and in _Synaptomys_. The baculum in these two genera in comparison to that in _Myopus_ (as figured by Ognev, 1948:512) and _Lemmus_ has a slenderer stalk and smaller digital ossifications or none at all. The baculum in the genera of lemmings increases in robustness and the development of processes from _Dicrostonyx_, to _Synaptomys_, to _Myopus_, to _Lemmus_--the same order outlined above for total of specialization. The two extremes in this series are near the extremes of variation in bacula to be found in all microtines. The baculum in lemmings as a group cannot then be considered more primitive than in voles as a group, although the voles are usually considered to be more advanced. The situation in the voles, as we shall see, casts a different light on the matter.