A total of 85 specimens of the gill-breathing proso-branch snail Probythinella lacustris (Baker) were collected from 3 localities of Late Pleistocene lake deposits in the Gentile Valley of southeastern Idaho, by Robert C. Bright, of the University of Minnesota. These specimens represent the first known (fossil or Recent) occurrence of this species from the W. side of the continental divide and indicate that Probythinella lacustris possessed a considerably greater geographic distribution during the Late Pleistocene than it does at the present time. Two of the localities have C-14 dates based on shell material; of 27,000+ or -900 years B.P. and 34,0001+ or -1600 years B.P.

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