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).