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Table 6. Comparison of size (mm SL) relative to mouth position and lower lip lobe separation for metalarvae (M) and juveniles (J, ~40 mm SL) of Upper Colorado River Basin catostomids. Size is preceded by initials for the applicable developmental intervals; "r" indicates that the condition is rare. Catostomus Catostomus Catostomus Calostomus Catostomus Catostomus Xyrauchen Character ardens catostomus commersoni discobolus latipinnis platyrhynchus texanus Mouth position Tenninal, above bottom of eye M~19 Md8 M ~17 MJ ~25 MJ ~25 Low tenninal, at or below bottom of eye MJdI MJ ~34 MJall M~19 MJall MJ ~25 MJall Subtenninal, low and not most anterior portion of snout h23 MJ ~18 hl9 MJ ~18 MJ ~22 h23 J ~32 Lower lip lobes, median separation Indistinct M ~18 Md5 Md8 MJ ~25 M ~22 Well separated MJ ~25 M 15-18 MJ ~31 MJ ~28 M ~20 MJ all MJd7 Slightly separated MJ ~18 MJ 18-37 MJ 17-31 h22 MJall h23 MJ 20-37 None, adjacent J ~22(r) MJ ~18 MJ ~17 h22 MJ ~22 J ~26(r) MJ ~20 in longnose and flannelmouth suckers with all specimens over 18 or 20 mm SL, respectively, having either slightly separated or adjacent lip lobes (Figs. 61-63, 104-105). The presence or absence of notches at the comers of the mouth is diagnostic for juveniles as well as adults. For bluehead and mountain suckers, the notches are present and distinctly separate the upper and lower lips (Figs. 48 and 49). For the other species, distinct notches do not develop and the upper and lower lips are more-or-Iess smoothly joined (Figs. 62 and 63). Osteological features. - Osteological features can be conclusively diagnostic for late metalarvae and juveniles of razorback sucker, subgenus Pantosteus, and subgenus Catostomus. Unfortunately these characters, as well as vertebra counts discussed under meristics, require that specimens be cleared and preferably stained for bone (or that the structures of interest be otherwise exposed). They are therefore best used to confirm or refine identities based on more external characters for which special preparation is not required. The frontoparietal fontanelle (opening between the frontal and parietal bones-covered with connective tissue) and first interneural bone are observable in some late postflexion meso larvae whereas the remaining skeletal characters considered herein 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 differences. As the bones of the skull form, an oval to rectangular fontanelle approximately half as wide as long forms in postflexion mesolarvae and small metalarvae. By 20 mm SL, the fontanelle narrows to a more rectangular shape and maximum width is less than 50% of maximum length for all but razorback and longnose suckers (Table 7, Fig. 7). Beyond 20 rom SL, fontanelle length increases proportionately with body length, but width and shape vary with species. Width generally also increases in razorback sucker, maintaining a more-or-Iess oval shape, decreases in mountain sucker, and remains relatively constant in the others (greatest in longnose sucker and least in bluehead sucker). For specimens 26-46 rom SL, fontanelle width remains at least 48% oflength in most razorback sucker (rarely as low as 43%), drops to less than 25% in mountain sucker, and ranges from 25 to 47% in the others (generally greatest in 33