I determined the distribution of limpets (Gastropoda: Ancylidae: Ferrissia) in the Colorado River and Rio Grande basins by handpicking specimens from the undersides of rocks and vegetation at 16 sites from a total of 495 surveyed sites in Colorado, Utah, New Mexico, and Arizona. Shell morphology, including morphometries, was compared to species holotype descriptions and museum lots. Ferrissia rivularis occurred in the upper Colorado, Gunnison, and San Juan rivers, and in the upper Rio Grande basin. Ferrissia walkeri, hereafter called the Walker morph, occurred only in the Gila River basin of Arizona and New Mexico. I reviewed the problems of classification of the Walker morph as being either F. walkeri, described from Arkansas and noted in Michigan and Baja California Sur, or Ferrissia fragilis, described from California and noted from the eastern United States. The Walker morph is identified by its vertical trending protoconch, which lies well within the right posterior quadrant, with the apex of the shell within the posterior third, and the anterior edge of the protoconch lying posterior to and below the apex. The importance of the Gila River Ferrissia population is discussed in reference to the often-proposed paleoaquatic fauna route between the eastern United States and California.
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Vol. 70 • No. 4