The primary ossification is in the stalk. The secondary ossification is in the median process except in _Lagurus_ (Dearden, 1958:551) and some individuals of _Neofiber_ (see account on page 258). Tertiary centers of ossification are in the lateral processes. The primary ossification is present at an early age and subsequently increases in size and solidity. The secondary and tertiary ossifications are progressively more common in older voles. The increase in degree of ossification of all parts continues after sexual maturity is reached. Individual variation and variation with age in the baculum of _Microtus pennsylvanicus_ have been illustrated by Hamilton (1946:380). Figures 14, 15, and 17 illustrate variation with size, which is correlated with age, and also illustrate individual variation. The three bacula are from adult voles having testes that measured 15, 16 and 16 mm. in length, respectively. Each vole was trapped in late June. The total lengths in millimeters of the three voles are 172, 167, and 181; weights are 55, 52.4, and 65.5 grams. I judge that the greater size of the stalk and the better developed base shown in Figure 17 than in Figure 15 are illustrative of age variation; the difference in the size of the lateral digitate processes is, in this case, attributable to individual variation. Differences in the distal end of the baculum in Figures 42 and 43, show individual variation also. Figures 35 and 36 represent two different subspecies; different individuals of _M. mexicanus mogollonensis_, however, exhibit individual variation of the same degree.
Hall and Cockrum (1953) list 44 species of microtines in North America. At least twelve of these are insular or local forms perhaps derived from some other species; for example _Microtus coronarius_, an insular form derived from _Microtus longicaudus_; _Microtus provectus_, considered by Chamberlain (1954:587) and by Wheeler (1956:176) as a subspecies of _Microtus pennsylvanicus_; and _Microtus ludovicianus_, a close relative of _Microtus ochrogaster_.
All North American genera have been studied. Of the genus _Microtus_ in North America, all subgenera but _Orthriomys_ and all species but the following nine, have been studied: _M. (Orthriomys) umbrosus_, the insular _M. (Stenocranius) abbreviatus_, _M. (Microtus) breweri_, _M. (Microtus) nesophilus_, _M._