The eastern species _Sorex longirostris_ has many similarities with shrews of the _ornatus-vagrans_ stock. _S. l. longirostris_ is close in many ways to _S. nanus_. Indeed, the differences between the species _S. nanus_, _S. ornatus_, and _S. longirostris_ seem to me to be of the same magnitude and indicate a similar period of differentiation from a common ancestor. The ancestor of _S. longirostris_ may have gained access to the eastern United States in the Illinoian Age _via_ the northern Great Plains south of the glacial boundary (fig. 7). The ancestor of _Sorex veraepacis_ of southern Mexico probably reached that area in Illinoian time as part of the ancestral _vagrans_ stock and probably attained its differentiation during the Sangamonian interval.
All the kinds of shrews so far discussed, including the _S. vagrans_ complex, might thus be thought of as having had a common ancestor in the Illinoian Age. This entire group of shrews has the third unicuspid smaller than the fourth, a pigmented ridge from the apex to the cingulum of each upper unicuspid, and, in most individuals, lacks a post-mandibular foramen in the lower jaw (Findley, 1953:636-637). The pigment is not always prominent in _S. longirostris_.
Two other species of North American shrews,_ Sorex palustris_, the water shrew, and _Sorex bendiri_, the marsh shrew, show these three characters to a greater or lesser degree, and it seems that these two species and the _vagrans-ornatus-veraepacis_ group had a common ancestor, probably before Illinoian time for reasons stated beyond. I judge, however, that far from being subgenerically distinct as they have been considered to be, _S. palustris_ and _S. bendiri_ are actually closely related species of the same subgenus and may have differentiated from one another because of separation into eastern (_palustris_) and western (_bendiri_) segments in the Sangamonian interval, much as has been postulated concerning the eastern and western stocks of _Sorex vagrans_. Indeed, Jackson (1928:192) has noted that in the Pacific northwest the characters of the two kinds approach one another and become differences of degree only.
The widespread species _Sorex cinereus_ resembles all the foregoing species in the ridges on the unicuspid teeth and in the lack of a post-mandibular foramen, but differs from those other species in having the third upper unicuspid larger than the fourth. The subspecies _S. cinereus ohionensis_, however, often has the sizes of these teeth reversed. With _S. cinereus_ I include _S. preblei_ (eastern Oregon) and _S. lyelli_ (Sierra Nevada), both obviously closely related to _cinereus_ as Jackson (1928:37) recognized when he included them in the _cinereus_ group. _Sorex milleri_ (Coahuila and central western Nuevo Leon) seems to me to resemble _S. cinereus_ more than it does other species of North American _Sorex_, and I judge that it also belongs to the _cinereus_ group. _Sorex cinereus_ and its close relatives seem more closely related to the species which have thus far been discussed than they do to such other North American species as _S. arcticus_, _S. fumeus_, _S. trowbridgii_, _S. merriami_, and the members of the _S. saussurei_ group; most of these five species last mentioned possess a post-mandibular foramen, lack pigmented unicuspid ridges, and have the third unicuspid larger than the fourth. Because of the morphological resemblances mentioned above, it seems likely to me that _S. cinereus_ and the _vagrans-ornatus-veraepacis-palustris_ complex had a common ancestor in early Pleistocene time. _Sorex cinereus_ has recently been considered to be conspecific with the Old World_ S. caecutiens_ Laxmann (Van den Brink, 1953) which name, being the older, would apply to the circumpolar species.
