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.