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and timing of fin development, fin lengths, and basal lengths of the dorsal and anal fins are also useful. The median finfold, one of the most obvious structures in proto larvae and meso- larvae, is a thin, erect, medial fold of tissue that originates on the dorsal surface usually well behind the head. It extends posteriorly to and around the end of the notochord, then anteriorly along the ventral surface to the posterior margin of the vent. During the meso larval phase, the soft-rayed portions of the median fins (dorsal, anal, and caudal) differentiate from this finfold. As the median fins develop, the finfold dimin- ishes and recedes before and between the fins until it is no longer apparent during or near the end of the metalarval phase. The preanal finfold is a second median fold of tissue that extends forward from the vent. In most fishes the preanal finfold is completely separated from the ventral portion ofthe median finfold by the vent. But in burbot (Lota Iota), and its marine relatives (Gadidae, codfishes), the preanal finfold is initially continuous with the median finfold and only later are the finfolds entirely separated by the vent (vent initially opens through right side of finfold). The preanal finfold mayor may not be present upon hatching, depending upon size and shape of the yolk sac. In cypriniform fishes, it is typically absent or barely apparent upon hatching. As yolk is consumed and the yolk sac decreases in size prior to hatching or during the proto larval phase, a small preanal finfold appears just anterior to the vent. As more yolk is consumed and the larva grows, the preanal finfold enlarges and extends anteriorly. Ultimately, its origin lies anterior to that of the dorsal portion of the median finfold. The preanal finfold remains prominent throughout the meso larval phase, then slowly diminishes and recedes in a posterior direction during the metalarval phase. It is typically the last finfold to be absorbed or lost. The caudal fin is the first fin to differen- tiate from the median finfold in cypriniform and most other fishes with homocercal tails. The portion of the finfold involved first thickens along the ventral side of the posterior end of the notochord and begins to differentiate into the hypural elements of the caudal skeleton. Im- mediately thereafter, the first caudal-fin rays appear (beginning of flexion meso larval phase) and the posterior portion of the notochord begins to bend or flex upward. Be careful not to confuse striations or folds in the finfold with developing fin rays. As the fin develops and the notochord continues to flex upward, the hypurals and developing caudal-fin rays, all ventral to the notochord, move to a posterior or terminal position. The first principal rays are medial and subsequent principal rays form progressively above and 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 comple- ment of principal rays becomes evident and notochord flexion is completed (beginning of postflexion mesolarval phase). The number of principal caudal-fin 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 (nine and nine respectively). 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. Accord- ingly, counts of rudimentary 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-fin rays. Tissue fIrst aggregates in vicinity of the future fin, 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 meso larval phase. Anterior principal fin rays develop first and sub- sequent 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 fin rays are added in an anterior direction. 11