Other species of _Sorex_ now occurring in North America differ from _Otisorex_ in having the 3rd unicuspid usually larger than 4th, in lacking a pigmented ridge from the apices to the cingula of the upper unicuspids, and in usually possessing a well-developed post-mandibular foramen. Exceptions to the last mentioned character are _S. fumeus_ and _S. dispar_. The subgenus _Sorex_ in North America should include only the following species: _S. jacksoni_, _S. tundrensis_, _S. arcticus_, _S. gaspensis_, _S. dispar_, _S. fumeus_, _S. trowbridgii_, _S. merriami_, and all the members of the Mexican _S. saussurei_ group. The subgenera _Otisorex_ and _Sorex_ probably separated in early Pleistocene or late Pliocene. _Sorex_ is unknown in North America earlier than the late Pliocene (Simpson, 1945:51). In the genus _Microsorex_ the characters of the subgenus _Otisorex_ are carried to an extreme; the unicuspid ridges are prominent and end in distinct cusplets, and the 3rd unicuspid is not merely smaller than the 4th, but is reduced almost to the vanishing point. In addition, the post-mandibular foramen is absent. Although it is closer structurally to _Otisorex_ than to _Sorex_, the recognition of _Microsorex_ as a distinct genus seems warranted. Figure 15 is intended to represent graphically some of the relationships discussed above. It must be re-emphasized that much of it is purely speculative, especially as regards actual time when various separations took place. It will be noted that I have indicated most separations as having taken place in interglacial ages. They are generally regarded as periods of warmth and aridity and, therefore, probably are times of segmentation of the ranges of boreal mammals and hence times exceptionally favorable to the process of speciation. Glacial ages, characterized by extensive and continuous areas of boreal habitat, probably were times of relatively unrestricted gene flow between many populations of boreal mammals and hence not favorable to rapid speciation. =Sorex vagrans= Wandering Shrew