The microtine mousePitymys meadensisHibbard, a species indicative of the late Irvingtonian mammal age in the United States, is present in a fauna of Rancholabrean age in the Valley of Mexico at El Tajo de Tequixquiac. It is closely related to the livingPitymys quasiaterof Sierra Madre Oriental. Following the reasoning of Van der Meulen, the species group ofPitymys pinetorumis recognized as a distinct North American lineage and the North American speciesP. meadensis(fossil),P. quasiater, andP. nemoralisare considered more closely related to Old World species ofPitymys, some of which have been placed in the genusNeodon. Pitymysand its immediate ancestorAllophaiomysare known from Europe and Asia in early Irvingtonian time.Allophaiomysalso dispersed southward from Beringia into North America in early Irvingtonian time, where thepinetorumspecies group developed from it in late Irvingtonian time. Thepinetorumspecies group seems always to have been restricted to the dry steppes of the northern Great Plains and to the forests well east of the Mississippi River. Thequasiaterspecies group dispersed from Beringia into North America in late Irvingtonian time and now survives partly in geographic asylum in central Mexico and the southern United States, as well as in southern Asia and southern Europe. It also survives partly in ecologic asylum withMicrotus, a more dominant microtine genus, in the more northerly parts of the United States and Europe. The genus is distinctly separate fromMicrotusand is here recognized as having full generic status.