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species. Some white, flannelmouth, and razorback suckers have low terminal mouths throughout the metalarval phase and early juvenile period, at least up to 40 mm SL (Figs. 90 and 91). The fIrst subterminal mouths appear as early as 18 mm SL for bluehead metalarvae and as late as 32 mm SL for razorback juveniles. All bluehead sucker juveniles and metalarvae over 19 mm SL have subterminal mouths (Figs. 47-49). Likewise for all mountain sucker greater than 25 mm SL and Utah sucker larger than 31 mm SL. The median cleft of the lower lip divides the lip of these suckers into two distinct lobes. The cleft is deep in most suckers but is shallow in bluehead and mountain suckers in which it is bridged by a few rows of papillae. Once the lower lips are sufficiently formed to distinguish two lobes, the lower lip lobes of most metalarvae and some juveniles of all species are well separated. This separ- ation continues for some Utah, white, and bluehead sucker up to 25 to 31 mm SL (Figs. 47 and 48), some razorback suckers up to at least 37 mm SL, and many mountain suckers to at least 40 mm SL (Figs. 75-77). The gap closes much more rapidly for flannelmouth sucker with all specimens over 20 mm SL having either adjacent or slightly separated lip lobes (Figs. 61-63). Metalarvae of neither mountain nor bluehead sucker have adjacent or only slightly separated lip lobes; only their juveniles have these traits. The presence or absence of notches at the corners of the mouth is diagnostic for juveniles as well as adults. For bluehead and mountain suckers, the notches are present and separate the upper and lower lips (Figs. 48 and 49). For the other species, distinct notches are not present and the upper and lower lips are more-or-Iess smoothly joined (Figs. 62 and 63). Osteological features can be conclusively diagnostic for late metalarvae and juveniles of razorback sucker, subgenusPantosteus, and subgenus Catostomus. Unfortunately these characters, as well as vertebra counts dis- cussed under meristics, require that speci- mens be cleared and preferably stained for bone (or that the structures of interest be otherwise exposed). They are therefore best used to confIrm or refme identities based on more external characters for which special preparation is not required. The fronto- parietal fontanelle (opening between the frontal and parietal bones covered with con- nective tissue) and first interneural bone are observable in some late postflexion meso- larvae while the remaining skeletal characters considered are applicable only to larger metalarvae and juveniles (Fig. 6). Adult descriptions suggest that more detailed study of larval and early juvenile skeletons might reveal additional skeletal differences, but these are probably the more obvious. As the bones of the skull form, an oval to rectangular fontanelle approximately half as wide as long forms in postflexion meso- larvae and small metalarvae. By 20 mm SL, the fontanelle narrows to a rectangular shape and maximum width less than 45% of maxi- mum length for all but razorback sucker Table 6. Comparison of size (mm SL) relative to mouth position and lower lip lobe separation for metalarvae (M) and juveniles (J, ~ mm SL) of Upper Colorado River Basin catostomids. Size is preceded by initials for the applicable developmental inteJVals; "r" indicates that the condition is rare. Catostomus Catostomus Catostomus Catostomus Catostomus Xyrauchen Character ardens commersoni discobolus latipinnis platyrhynchus texanus Mouth position Terminal, above bottom of eye M s19 M s18 M s17 MJ s25 MJ s25 Low terminal, at or below bottom of eye MJ s31 MJ all M s19 MJ all MJ s25 MJall Subterminal, low and not most anterior portion of snout J~23 J ~19 MJ ~18 MJ~ h23 h32 Lower lip lobes, median separation Indistinct M s18 M s18 MJ s25 Ms22 Well separated MJs25 MJ s31 MJ s28 M s20 MJ all MJ s37 Slightly separated MJ ~18 MJ 17-31 h22 MJ all J~23 MJ 20-37 None, adjacent J ~22(r) MJ ~17 J~2 MJ ~22 J ~(r) MJ~ 32