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larvae typically has 46 to 48 total vertebrae or myomeres. The number of myomeres to the vent is typically 37 or greater for bluehead, flannelmouth, and razorback sucker, 37 or fewer for Utah sucker, and less than 37 for mountain sucker; white sucker larvae typically have 35 to 38 myomeres to the vent. For protolarvae and flexion mesolarvae most diagnostically useful measures relate to the amount of yolk remaining as the fish grow (Table 3). By the end of the protolarva phase, mountain and razorback suckers con- sume most but not all of their yolk and blue- head and especially flannelmouth suckers still retain about half of their original yolk supply by the end of the protolarva phase. All suc- kers except some flannelmouth complete yolk absorption by the end of the flexion meso- larva phase. For late postflexion mesolarvae, meta- larvae, and juveniles most diagnostic mea- sures relate to the size and position of the dorsal fm. The length of the dorsal fin (from origin of the fm to its most distal margin) and length of the base of the fm correlate well with the number of principal fin rays. As would be expected, these measures are great- est for razorback sucker and least for moun- tain sucker, but not much larger for bluehead and white suckers. Length to the insertion of the dorsal fm is also greatest for razorback and least for mountain sucker, while length to the origin of the fin is greatest for bluehead sucker, followed by white and mountain suc- kers, and least for flannelmouth and razor- back suckers. Among the remaining measures, only eye diameter is useful for all developmental inter- vals. As protolarvae, mountain sucker gener- ally have both the greatest eye diameters and head lengths (measured to the origin of the pectoral fin bud) relative to standard (noto- chord) length. Bluehead and flannelmouth protolarvae typically have the smallest eyes and heads. For subsequent developmental intervals, differences in eye diameter are best considered as a percentage of head length. For these later stages Utah sucker usually have the largest eyes while flannelmouth sucker continue to average the smallest eyes, although not by much. Head length among juveniles is often greatest for razorback and white suckers and least for bluehead, flannel- mouth, and mountain suckers. In addition to dorsal fin lengths discussed above, pectoral and caudal fin lengths are also useful for specific developmental inter- vals. Pectoral fm length is sufficiently diag- nostic only for protolarvae, and then only with respect to the maximum values for which it is greatest for white, mountain, and razorback suckers and least for Utah and bluehead suc-kers. Caudal fm length is sufficiently diag-nostic only for metalarvae and juveniles; for both intervals, caudal fm length is least for mountain sucker while it is greatest for razor-back metalarvae and both razorback and Utah sucker juveniles. Lengths from snout to pelvic fm origin and posterior margin of the vent, and body depth at the origin of the dorsal fm are the only remaining length characters considered sufficiently diagnostic to include in Table 3. Snout to pelvic fm origin lengths are appli- cable only to postflexion mesolarvae and later intervals and, like lengths to the origin of the dorsal fin, are typically greatest for bluehead sucker and least for flannelmouth sucker metalarvae and juveniles, thereby maintaining the pelvic fin origins a more-or-Iess similar horizontal distance behind dorsal fm origins. For postflexion mesolarvae, length to origin of the pelvic fm is least for Utah sucker but only slightly larger for flannelmouth and razorback suckers. Snout to vent lengths are greatest for Utah and razorback sucker meso- larvae and razorback juveniles. Body depth measured at the origin of the dorsal fin reflects the amount of yolk remain- ing in protolarvae and meso larvae, as noted above, but reflects the health or condition of the fish for later stages and, especially for the larger juveniles, differences in structural depth. The upper end of the range for this measure is notably greater for razorback juveniles and is probably due, at least in part, to enlarging interneural bones behind the head which will eventually form the distinc- tive predorsal "razor" or keel of older juven- iles and adults. Pigmentation: Capture of these suckers prior to initial eye and body pigmentation is rare. If not pigmented at hatching, at least the eyes and some body pigmentation is usu- ally evident by emergence from the spawning substrate. White and mountain suckers are usually well pigmented by 9 mm SL and Utah, bluehead, and flannelmouth suckers by 11 mm SL (Table 4). Pigmentation through- out early development is generally lightest for flannelmouth and especially razorback suckers. 27