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Fig. 7. Frontoparietal fontanelles of early juveniles. Top -- Xyrauchen texanus; wide and oval. Bottom- Catostomus species; moderately wide to narrow and rectangular. must be examined to determine if fontanelle closure is typical of mountain sucker popu- lations in the Upper Colorado River Basin. The large, fan-shaped, fIrst interneural bone of razorback sucker metalarvae and juveniles over 16 mm SL readily distinguishes it from the other species (Fig. 8). By late in the metalarval phase, the smaller interne urals posterior to the fIrst also develop enlarged or flared tops. The interneurals eventually form the skeletal basis for the unique pre dorsal keel or "razor" of the razorback sucker (Fig. 94). By 20 mm SL, the fIrst interneural also segregates the remaining species according to subgenera. Subgenus Catostomus (Utah, white, and flannelmouth suckers) have moderate to large, anvil-shaped fIrst inter- neurals with moderate to long posterior extensions (especially long in flannelmouth sucker). Subgenus Pantosteus (bluehead and mountain suckers) have smaller, somewhat blocky fIrst interneurals with short to moderate posterior extensions. Position of mandibles relative to maxillae also are diagnostic for subgenus Pantosteus. For juveniles and metalarvae greater than 22 mm SL, the anterior margins of the mandi- bles are closer to the posterior ends of the maxillae in bluehead and mountain suckers (Fig. 9). For the other species, they are closer to the anterior ends of the maxillae. However, by about 40 mm SL, at least some flannelmouth suckers have anterior margins Fig. 8. Interneural bones of late metalarvae and early juveniles. Top - Xyrauchen texanus; first interneural large and fan-shaped; posterior inter-neurals well formed and flared dorsally. Middle -- subgenus Catostomus; first interneural moderate to large, anvil- shaped with prominent posterior extension. Bottom -- subgenus Pantosteus; first interneural smaller and more blocky with short to moderate posterior projec- tion. 34