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11. a. Anal rays 10-13, typically 11 or 12. Dorsal rays 8-10, typically 9. Myomeres 23-26 to the vent, typically 24-25, and 36-41 total, typ- ically 37-39. Dorsal fin origin far behind pelvic fin origin, nearly 6% Sl difference in position. Metalarvae as small as 10 mm Sl (11 mm TL). . Riahapdsonius balteatus. 12. a. Dorsal and anal rays typically la, rarely 9, 11 or 12. Myomeres, based on adult vent position and vertebra counts, probably 30 or less to the vent and 48-50 total. . . . . . . . . . . . . . Gila elegans (speculative, could be G. aypha or G. robusta). 13. a. Myomeres typically 21 or more to the dorsal fin origin, 31 or more to the vent, and 48 or more total. Anal fin rays typically 9, rarely 10. .......... PtyahoaheiZus Zuaius. 14. a. Anal rays usually 9, sometimes 10. . . . . . . GiZa robusta, possibly GiZa aypha. 15. a. Dorsal rays 15-16. Xyrauahen texanus. 16. a. Dorsal rays 14. . . . . . . . either Catostomus latipinnis or Xyrauahen texanus. 17. a. Peritoneum dark, well pigmented with melano- phores. Gut with one or two loops crossing from side to side. . . Catostomus disaobolus or Catostomus pla~Jrhynahus (subgenus Pantosteus; latter species usually restricted to headwater streams) . 18. a. Dorsal rays typically 12 or 13, with rare extremes of 11 and 14. Gut "S" shaped through- out metalarval phase. Ventral surface without a mid-ventral line of melanophores from pecto- ral fins to vent or with only a short series. . . . . . . . . . .. Catostomus Zatipinnis. 19. a. Eye diameter less than 6% SL. Body depth at vent from dorsal surface to ventral surface of the vent itself less than 7% Sl (Fuiman and Witman, 1979). .. Catostomus aorrunersoni. 20. a. Eye diameter greater than 6% SL. Body depth at vent, including vent, greater than 7% Sl (Fuiman and Witman, 1979). ......... . . . . . . . . . . . . Catostomus aatostomus. KEY TO THE METALARVAE 17 b. Dorsal and anal rays typically 9 or 10, rarely 11 or 12. Myomeres 26 or greater to the vent, typically 28 or greater, and 42 or greater total, typically 44 or greater. Dorsal fin origin less posterior to pelvic fin origin, only about 2-4% Sl difference in position. Metalarvae no smaller than 12 mm SL (14 mm Tl). ..... 12. b. Dorsal rays typically 9, rarely 8 or 10. Total myomeres typically less than 48 or if 48 or more, preanal 1T\Y0meres greater than 30. ..... 13. b. ~lyomeres typically 20 or less to the dorsal fin, 30 or less to the vent and 47 or less total. Anal fin rays typically 9 or 10. ...... 14. b. Anal rays usually 10, sometimes 9. ...... GiZa aypha, possibly GiZa robusta (G. aypha not expected outside canyon areas). b. Dorsal rays 14 or less. 16. . . eo . . . . . . . b. Dorsal rays 13 or less. 17. . . . . . . . . . . b. Peritoneum essentially unpigmented on ventral surfaces. Gut either "S" shaped or with one, rarely two, loops crossing from side to side in later metalarvae. . .18 (subgenus Catostomus). b. Dorsal rays typically 10 or 11, with rare extremes of 9 and 13. Gut "S" shaped in earlier metalarvae and with one, rarely two, loops cross- ing from side to side in later metalarvae. Ven- tral surface with distinctive mid-ventral line of melanophores from pectoral fins to vent, some- times incomplete. . . . . . . . . . . . 19. b. Eye diameter 6% SL or greater. Body depth at vent, including vent, 7% Sl or greater. 20. b. Eye diameter about 6% Sl. Body depth at vent, including vent, about 7-8% Sl. . . . . . . Catostomus aatostomus or Catostomus aommersoni.