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.

The voles, Tribe Microti, were considered by Hinton (1926:40) to be more advanced than the lemmings because the incisors of the voles are longer and the root of their last lower molar is lingual to the root of the incisor. Hinton thought also that the murine ancestors of microtines had shorter incisors and that the backward extension of the incisors in the voles is a more ancient feature than the hypsodonty of the molars. A trend in the molar teeth has been toward greater hypsodonty. The voles in which the molars are least hypsodont are thus considered primitive. These include the living genera Clethrionomys, Phenacomys, Ondatra, Dolomys, Ellobius, and Prometheomys. Therefore, the baculum, in these assumedly primitive genera, would be expected to resemble the baculum in the lemmings or at least the most primitive lemmings. This is not the case.

The bacula that I have examined of Clethrionomys and Phenacomys have well-developed digital ossifications. In this they resemble the baculum of the genus Lemmus, the most advanced genus of lemmings according to Hinton. The baculum of Dolomys has not been studied. The baculum in Ondatra, and in Prometheomys as illustrated by Ognev (1948:552), also possesses well-developed processes. The baculum of Ellobius is small and lacks processes (as figured by Ognev, 1950:662). No ossification was found in a single specimen of Ellobius examined by me although the entire glans penis was removed and cleared without dissection. So far as known then, with the exception of Ellobius and Phenacomys longicaudus (Dearden, 1958:547), the primitive microtines having rooted molars possess bacula having three well-developed ossified processes.

Voles of the genus Microtus vary in the structure of the baculum almost as much as do the lemmings. Within the single subgenus Microtus some individuals of Microtus mexicanus, for example, have minute ossified lateral processes and other individuals lack these processes; Microtus pennsylvanicus and some other species have proportionately large lateral ossifications. If the well-developed condition of the baculum in the microtines having rooted molars is primitive, then within the genus Microtus those species having well-developed bacula may be considered primitive.

The genera Lagurus and Neofiber have moderately developed or well-developed lateral processes. Neofiber exhibits a tendency, not prominent elsewhere, to have a proportionately smaller median process rather than reduced lateral processes.

American species of Microtus (genus and subgenus) that have moderately- to well-developed ossified lateral processes are M. townsendii, M. oeconomus, M. pennsylvanicus, M. montanus, and M. chrotorrhinus. Microtus of other subgenera having this type of baculum include M. (Herpetomys) guatemalensis, M. (Chilotus) oregoni, and M. (Chionomys) longicaudus.

American species of Microtus (genus and subgenus) in which the lateral ossifications are weakly developed or absent (although cartilaginous lateral processes are present) include M. mexicanus and M. californicus. In other subgenera, species of Microtus having reduced lateral ossifications are M. (Pedomys) ochrogaster, M. (Pitymys) pinetorum, M. (Pitymys) parvulus, M. (Pitymys) quasiater, M. (Arvicola) richardsoni, and M. (Stenocranius) miurus.

The microtines are essentially holarctic in distribution. Both of the tribes, the lemmings and the voles, as well as primitive representatives of each tribe (not considering Ellobius) occur in both the Old World and New World. It is not certain on which continent (or continents) the Microtinae first differentiated. It is certain, however, that at various times, both early and late in the evolution of the subfamily, representatives have crossed from Eurasia to North America or vice versa. Each of 10 or more microtines in the New World is more closely related to some microtine in the Old World than to any other microtine in the New World.

The similarities or differences in the baculum in Old World and New World representatives placed in the same genus or subgenus, or thought to be "companion species" have been commented upon in accounts of Lemmus, Dicrostonyx, Clethrionomys, Lagurus, Arvicola, Stenocranius, Chilotus, Chionomys, Pitymys, and in accounts of Microtus agrestis as compared with M. pennsylvanicus, and Microtus oeconomus (both Old World and New World).

The baculum in the Microtinae more closely resembles the baculum in the Cricetinae of the Old World than in the Murinae, or than in any other rodents known to me. This resemblance suggests relationship between Microtinae and Cricetinae.