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Notiosorex shrews (Soricomorpha: Soricidae) have a fossil history that extends to the Miocene (i.e., mid-Hemphillian). In the intervening 6.69 million years, N. repenningi, N. jacksoni, and what heretofore was considered N. crawfordi existed with varying degrees of overlap in occurrence timewise and in geographic distribution in areas of the southwestern United States and northern Mexico. Examination of unusually small fossil specimens of Notiosorex indicated the presence of another species referable to this genus present in late Pleistocene- (i.e., early Wisconsinan) to Holocene-aged deposits. Based on statistical analyses, these specimens were as distinct from any of the other fossil or extant species of Notiosorex as any other pair of species within the genus. I referred these specimens to the new species N. harrisi. Analyses also indicted fossils heretofore considered N. crawfordi were equally distinct from other forms of Notiosorex. I referred them to the new species N. dalquesti. Individuals referred to the fossil species of Notiosorex range in size from the smaller N. harrisi, to N. dalquesti, to N. jacksoni, to the larger N. repenningi. This size differential could be related to changing climatic conditions with the concomitant ecological succession that occurred, which provided a driving force for speciation within the genus. The changing climate and unique morphological and physiological adaptations allowed fossil taxa of Notiosorex to expand their distributions into areas subsequently less suitable for other forms of shrews. Present-day notiosoricini shrews occur throughout much of the southwestern United States and northern Mexico in a variety of habitats in association with rough, rocky terrain. Thus, if climate, even indirectly, was the driving force for speciation among fossil taxa of Notiosorex, other forces must have acted on the extant species N. cockrumi, N. crawfordi, N. evotis, and N. villai. [ABSTRACT FROM AUTHOR]