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12 LARVAL CHARACTERISTICS cypriniforms typically appear mid-laterally on the posterior half of the body and from there spread anteriorly, dorsally and ventrally toward adult coverage. The scales of larger-scaled species are sometimes obvious by late in the meta1arva1 phase and may be used to separate or help distinguish certain species or genera. Morphology: The shape or form of larvae and specific anatomical structures, which change as the fish grow and develop, provide some of the most obvious characters for identification purposes, particularly at the family and SUbfamily levels, occasionally at the species level. Much of this shape or form-related information can be quantified to some degree via proportional measurements or morphometrics. The Shape and form of structures such as the gut, air bladder, yolk sac, and mouth, especially as they change during development, can be diagnostic. MOTphometria aata emphasizes the relative position and relative size of various body components and body dimensions, and may be critical to species identification of certain larvae. Such measurements may be allometric, changing in proportion as the fish grow; thus morphometric data should be related to size, at least for protolarvae and mesolarvae. Some morphometric data, particularly body depths and widths, may be directly affected by the condition of individual specimens and the volume and form of food items in their digestive tracts. The source of the specimens and the nature of the solution in which they are stored should also be considered in the use of this data. Shrinkage and deformation are greater in alCOhol than in formalin. Morphometric data in this guide are reported as a percentage of standard length. Use of standard length avoids the allometric influence of caudal fin growth included in percentages based on total length. As explained later (Methods), conversion of certain data to percent total length for comparison with other works is relatively simple. Prior to hypura1 plate formation and completion of notochord flexion, herein correlated with the acquisition of the adult complement of principal caudal fin rays, standard length is defined as notochord length (snout to the posterior end of the notochord). Thereafter, it is defined as the length from the anterior margin of the snout to the most posterior margin of the hypura1 plates (usually the superior plate or hypura1s). Use of notochord length for proto1arvae and early meso1arvae gives the appearance of greater allometric growth differences than may really exist, at least in comparison with subsequent measures based on the posterior margin of the hypura1 plates. This undesirable effect is a result of the upward bending or flexing of the notochord and the switch from use of the end of the notochord to the posterior margin of the hypura1s as the basis for length measurement. These factors must be taken into account when reviewing the morphometric data given herein. Measurement of body lengths and various parts thereof, in contrast to the procedures recommended by Hubbs and Lagler (1958) for larger juveniles and adults, is generally done along a line parallel to the horizontal axis of the fish. Exceptions are fin lengths, which in studies conducted for this guide were measured from the origin of the fin base to the most distal margin of the fin rays. Typical measures include total, standard, snout-to-vent preanal, predorsa1, prepelvic, head, eye, snout and fin lengths. Snout-to-vent length, Which is measured to the posterior margin of the vent or anus, reflects the position of the vent. The term preanal length should be reserved specifically for the length measure from the snout to the origin of the anal fin; in many fishes, including the cypriniforms, the latter point is often the same or nearly the same as the posterior margin of the vent. The snout-to-vent length is a primary diagnostic character for many species, especially at the family and sometimes subfamily level. Except for most larvae of the common carp (Gyprinus aarpio) and an occasional meso1arva of the Colorado squawfish (PtyahoaheiZus Zuaius), cyprinid larvae in the Upper Colorado River System are readily differentiated from catostomid larvae by snout-to-vent lengths of less than 72% SL. Head length is typically measured to the posterior margin of the operculum in juveniles and adults, but the operculum may be absent or incom- plete throughout much of the larval period. Accordingly, many biologists have redefined head length to be measured to the posterior end of the auditory vesicle or the anterior or posterior margin of the c1eithrum, one of the first bones to ossify in fish larvae (Berry and Richards 1973). Unfortunately, the auditory vesicle and c1eithrum are not always easily observed, especially later in larval development. Also, resultant measures from the auditory vesicle are considerably anterior to the eventual posterior margin of the operculum. Snyder et a 1. (1977) and Snyder and Doug1 as (1978) measured larval head length to the anterior insertion or origin of the pectoral fin. The base of the pectoral fin is readily observed throughout the larval period (except in the few species that hatch prior to pectoral bud formation), somewhat approxi- mates the pOSition of the c1eithrum (part of its supporting structure), and more nearly approximates the posterior margin of the operculum than does the posterior margin of the auditory vesicle. AccordinglY, head length is defined herein as the length from the anterior margin of the snout to the anterior-most margin or origin of the base of the pectoral fin and is used, for purposes of consistency, for juveniles as well as larvae. The measure is most precisely determined while examining the speci- men from above or below and, if necessary, holding the fin away from the body. Body depths and widths are measured in planes perpendicular to the horizontal axis of the fish. Many biologists report these as maximum or minimum measures (e.g., greatest head depth, greatest body depth, and least caudal peduncle depth). However, it seems more logical for comparative purposes to specify specifiC locations as standard reference points for such measures, as per Moser and Ahlstrom (1970), Fuiman (1978) and Snyder and Douglas (1978). Five speci fi c 10cati ons, four correspondi ng to specific length measurements, are used herein: 1) immediately posterior to the eyes, 2) origin of the pectoral fin, 3) origin of the dorsal fin, 4) ir.lJ1ediately posterior to the vent and 5) at the anterior margin (mid-lateral apex) of the most posterior myomere. Neither fins nor finfolds are included in depth measurements. Other mOTphoZogiaaZ aharaateps such as the position, size, and form of the mouth and the gut, and related changes can be among the more useful characters for identification to the species level. The size of the mouth, as well as its position and angle of inclination, and the form of specific