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gressively above below. Principal caudal fin rays articulate with hypural bones of the caudal structure and ultimately include all branched rays plus two adjacent unbranched rays, one above and one below the branched rays. Branching and segmentation of rays can be observed as or shortly after the full com- plement of principal rays becomes evident and notochord flexion is completed (begin- ning of postflexion mesolarval phase). The number of principal caudal fm rays is typically very stable within major groupings of fish. Cyprinids generally have 19 principal rays (ten based on superior hypurals and nine on inferior hypurals), and catostomids usually have 18 principal rays. Dorsal and ventral rudimentary rays of the caudal fin begin forming sequentially in an anterior direction immediately after all or nearly all principal caudal fin rays are formed. They are often the last group of fin rays among all fins to form their full adult complement. Accordingly, counts of rudi- mentary caudal fin rays are usually ignored in larval fish identification, but they may be of taxonomic value for juveniles and adults. The dorsal and anal fins, which typically form either simultaneously (many cyprinids) or dorsal first (most catostomids), usually begin development prior to attainment of the full complement of principal caudal fm rays. Tissue first aggregates in vicinity of the future fm, and basal structures or pterygiophores soon become evident. The latter structures permit limited use of dorsal and anal fin position and meristics about midway through the mesolarval phase. Anterior principal fm rays develop first and subsequent rays are added in a posterior direction. The first rudimentary fin rays (anterior to the principal rays) are frequently evident before all the principal fin rays form. Rudimentary fm rays are added in an anterior direction. The first or most anterior principal ray in both dorsal and anal fins remains unbranched while all other principal fin rays branch distally as or after ray segmentation becomes evident. The last or most posterior principal ray in each fm is considered to be divided at the base and therefore usually consists of two elements that, except for their close proximity and association with the same pterygiophore, might otherwise be considered as separate fm rays. Principal dorsal and anal fm ray counts between and within certain genera often vary sufficiently to be of use in identification at the species level, especially anal fm rays of cyprinids and dorsal fm rays of catostomids. Positions of dorsal fin origin (anterior attach- ment) and insertion (posterior attachment) relative to origin of pelvic fins or fm buds and the vent vary considerably among cyprinids and are useful in identification of genera or species. These position characters are more consistent among catostomids (e.g., dorsal fm origin is always well in advance of the pelvic fins), especially at subfamily level, and there- fore, are of less value in identification. The pelvic fins begin as buds before or upon transition to the metalarval phase. In cypriniform fishes, they originate in an abdo- minal position along each side of the preanal fmfold. They may erupt shortly after dorsal and anal fm development begins or be delay- ed until just before or shortly after all principal rays are present in the median fins. Pelvic rays begin to form shortly after the buds appear and the adult complement of rays quickly ensues. Among cypriniform fishes, pelvic ray counts are seldom used diagnostically. However, position of the pel- vic fins or fm buds, relative to other struc- tures, and their formation in the sequence of developmental events can be useful in identi- fication, especially among cyprinids. The pectoral fins typically begin as buds immediately behind the head in the late embryo. However, pectoral buds are not evi- dent in some cypriniform fishes until shortly after hatching. Though strongly striated and occasionally with membranous folds and breaks, they typically remain rayless in cypriniforms until late in the mesolarval phase when most of the principal median fin rays are present. With the exception of rudi- mentary caudal fm rays, the rays of pectoral fins are often the last to establish their full complement. For this reason and because the number of pectoral rays is usually rela- tively large and difficult to count without excision (especially the smaller ventral rays), pectoral fm ray counts are generally of little value in larval fish identification. 11