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Keys -- Upper Colorado River Basin Catostomids The continual change in available char- acte~s and c?aracter states and similarity of specIes durmg larval and early juvenile development are accommodated by separate keys for eggs, each of four larval phases and early juveniles up to 40 mm SL (about 50 mm TL). Users must begin by determining the devel?pmental interval of the specimen in question: embryo (egg), protolarva, flexion mesolarva, postflexion mesolarva metalarva , , or early (young-of-the-year) juvenile -- see prec~ding section on developmental interval termmology for complete defInitions. Users must then determine standard length since each key begins with size dependent charac- ers. Alt.hough the polychotomous keys are necessanly long and complex, often with multiple paths leading to the same conclu- ion, most specimens will require no more than several steps to reach a terminus. Supplemental keys based on selected skeletal characters are provided to confIrm or better resolve the identity of postflexion mesolarvae, metalarvae and juveniles (specimens must be cleared and preferably stained). A discussion of characters useful in iden- tifIcation of cypriniform larvae and diagrams of anatomical features, methods of counts and measures, gut loop phases, and selected skelet~ structures (Figs. 2-6) precede results. !he dIscussion of cypriniform characters mclu?~s criteria for distinguishing the families Cypnmdae and Catostomidae (see especially fIns and fInfolds, last paragraph on myo- meres, and .fIfth paragraph ?n morphology). A comparative summary of diagnostic charac- ters and species accounts immediately pre- cede and follow this section, respectively. The species accounts include full sets of dorsal, lateral, and ventral view drawings. Users of the keys should be familiar with this material before proceeding. The drawings in the species accounts are fre9uently referenced in the keys, especially t? illustrat.e specifIc patterns of melanophore pIgmentation. However, referenced fIgures are not necessarily of the species, develop- men~al state, or size appropriate to the portion of the key in which the reference is made. Each spot of black or brown pigment (melanin) represents a melanophore. In the keys, melanophores refer only to those with visible pigment (other melanophores may be present but lack pigment or sufficient amounts to be obvious under low power magnification) . These keys require the use of a low- power microscope and some means of m~asurin~ to ~he nearest 0.1 mm, e.g., pomted dial calIpers, graduated mechanical stage, or measuring eyepiece reticle with a stage micrometer disc or slide for calibration. If using the latter, be sure to set focus before calibration; any subsequent changes in focus or magnifIcation will require recalibration. If using a zoom microscope and eyepiece reti- cle, the scale can be calibrated for direct millimeter measurement. A 10 mm reticle scale in a lOX eyepiece focused on the speci- men (or stage micrometer) with precisely 1X of objective magnification will measure 10 mm (note that zoom control magnifIcation scales may not be accurate). At O.5X objec- tive magnification, the reticle scale will measure 20 mm (each 0.1 division then equals 0.2 mm). If the specimen is not too large, adjustment of magnifIcation to calibrate the reticle scale to standard length (Fig. 4) of the specimen will allow direct measurement of specifIc structures or dimensions as a percentage of standard length. Except for fm lengths, measurements are made parallel or perpendicular to the body axis (Fig. 4). Specimens must be completely s,ubmerged in fluid (usually water) and posi- tioned such that their body axis is horizontal. Flat pieces of glass can be used t.o prop or hold-down a specimen. For fIsh which taper from head to tail or have a pectoral fm pre- venting it from lying flat on the bottom of a dish, try suspending the head and part or most of the body over the edge of a suffI- ciently thick piece of glass. Lay another, smaller and thinner piece of glass over the tail to hold the specimen in position. Curved or bent specimens that can't be straightened or. gently. flattened under a cover slip or thicker pIece of glass can be measured in sections along the horizontal axis. Specimens larger than the range of the measuring devise also can be measured in sections. Measure- ment units are the nearest whole or tenth of a millimeter (e.g., a 12 mm specimen covers the range 11.50 to 12.49 mm, and a measure of 6.5 more precisely measures between 6.450 and 6.549). 37