Third unicuspid usually smaller than fourth; upper unicuspids usually with pigmented ridge extending from apices medially to cingula, uninterrupted by antero-posterior groove; post-mandibular foramen usually absent. Includes the species S. cinereus, S. longirostris, S. vagrans, S. ornatus, S. tenellus, S. trigonirostris, S. nanus, S. juncensis, S. willeti, S. sinuosus, S. veraepacis, S. palustris, S. bendiri, S. alaskanus, and S. pribilofensis.
| Figs. 11-14. | Characters of the subgenera Sorex and Otisorex. |
| Fig. 11. | Medial view of right ramus of Sorex (Otisorex) vagrans. × 14. |
| Fig. 12. | Medial view of right ramus of Sorex (Sorex) arcticus. × 14. |
| Fig. 13. | Anterior view of left second upper unicuspid of Sorex (Otisorex)vagrans. × 45. |
| Fig. 14. | Anterior view of left second upper unicuspid of Sorex (Sorex)arcticus. × 45. |
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