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RESULTS AND DISCUSSION Results are divided into three interrelated sections-Species Accounts, Comparative Sum- mary, and Computer-Interactive Key. For iden- tification purposes, users should become famil- iar with and use all three taxonomic tools. Athough prepared for use by UCRB biologists, these taxonomic tools, and other information provided herein, may also be useful to early life history investigators working else- where. Allowing for potential population differ- ences in developmental morphology, these descriptions and the key can be used for identi- fication of covered species wherever they may occur. For example, white and longnose sucker are common throughout much of Colorado (the only Catostomus species in east-slope drain- ages), and indeed much of North America. Bluehead, flannelmouth, and razorback suckers occur in portions of the Lower Colorado River Basin; bluehead sucker also in portions of the Bonneville Basin; and mountain sucker in mountainous regions throughout much of western United States and southwestern Canada. Where two or more of these species occur together and any other closely related sympatric species can be eliminated otherwise as possi- bilities, the computer-interactive key has the flexibility of being limited to just those species and effectively becoming a key for that region, site, or circumstance. Although 553 specimens were analyzed in detail for morphometrics and meristics, and hundreds more were documented for size, developmental state, skeletal characters, and pigmentation patterns, there are undoubtedly rare specimens with character extremes beyond the ranges recorded herein. Indeed, many ofthe descriptive data updates incorporated herein are verified character-state extensions reported or brought to our attention by users of the earlier edition of this guide (Snyder and Muth 1990). Because of the similarity among larvae of UCRB catostomids, the specific identity of some larvae will remain inconclusive or questionable after application of the key and diagnostic cri- teria provided herein. The identity of such spec- imens must be considered tentative and should be designated as such by appending a question mark ("?") to the most probable taxon name (e.g., "Xyrauchen texanus?", preferably with a foomote on other possibilities), or by leaving the identity at family level (e.g., "unidentified Catostomidae"), or genus (i.e., Catostomus sp.) if other genera can be eliminated. Some incon- clusive specimens may be hybrids. Hybridization among Colorado River System catostomids is well documented (e.g., Holden and Stalnaker 1975, Hubbs et al. 1943, Hubbs and Hubbs 1947, Hubbs and Miller 1953, McAda 1977, McAda and Wydoski 1980, Prewitt 1977, and Smith 1966). Intermediacy of characters for white X bluehead sucker hybrids as small as 25 mm SL and flannelmouth X blue- head sucker hybrids as small as 34 mm SL were documented by Hubbs et al. (1943) and Hubbs and Hubbs (1947) respectively. Based on the key or diagnostic criteria summarized herein, some hybrid metalarvae and early juveniles may be least tentatively identified as such by more experienced users, but because of fewer char- acters, hybrid proto1arvae and meso larvae, will likely be identified as the parental species they most closely resemble or remain questionable. 26