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merous morphological adaptations to swift-water and other features, warrants subspecific designation. Rhinichthys osculus moapae, endemic to the Moapa River in south- eastern Nevada, is closely related to R. o. velifer and R. o. yarrowi. Meristically, R. o. moapae is intermediate between R. o. veliter and R. o. yarrowi. Rhinichthys osculus moapae has a range of 53-72 lateral- line scales and a mean of 62.84; the holotype of R. o. velifer has 55 lateral-line scales; R. o. yarrowi characteristically has a range of 74- 83 scales in the lateral-line (Gilbert 1893; La Rivers 1962). About half of the specimens of R. o. yarrowi have a narrow premaxillary frenum, whereas all 3 specimens of R. o. velifer, reported in the original descrip- tion, have a frenum (Gilbert 1893); specimens of R. o. mOap'lB almost always have a well developed premaxillary frenum. Taxonomic data for R. o. veliter are, unfortunately, lacking as information for only 3 specimens has been reported. Presently, R. o. moapae is isolated from R. o. velifer by reduced flows in the White River system from Pluvial times, and from R. o. yarrowi by the flooding of the lower Moapa and Virgin Rivers by Lake Mead. Lake Mead forms a barrier to dispersal because it constitutes unsuit- able habitat for the speckled dace (Miller 1952). I wish to thank James E. Deacon for his guidance and review of this manuscript. Thanks also are due to Robert R. Miller and his staff for radiographs of R. o. moapae. Special thanks are due to my wife, Cindy, for making all work joyful through her help. Carl L. Hubbs is also gratefully acknowledged for his review. LITERATURE CITED GILBERT, C. H. 1893. Report on the fishes of the Death Valley expedition col- lected in southern California and Nevada in 1891, with descriptions of new species. N. Amer. Fauna 7: 229-234. HUBBS, C. L. 1943. Criteria for subspecies, species and genera, as determined by researches on fishes. Ann. New York Acad. Sci. 44: 109-121. . 1961. Isolating mechanisms in the speciation of fishes, in Vertebrate speciation! A University of Texas Symposium CW. F. Blair, ed.). Univ. Texas Press, Austin, pp. 5-23. -~, R. R. MILLER and L. C. HUBBS. 1974. Hydrographic history and relict fishes of the north-central Great Basin. California Acad. Sci. 7: 1-254. LA RIVERS, 1. 1962. Fishes and fisheries of Nevada. Nevada State Fish and Game Commission, Carson City, Nevada, 782 pp. MILLER, R. R. 1952. Bait fishes of the lower Colorado River from Lake Mead, Nevada, to Yuma, Arizona, with a key for their identification. California Fish and Game. 38: 7-42. 516