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easier, especially when examining many speci- mens, but they require appropriate X-ray equipment and a darkroom. Dunn (1983, 1984) reviewed use of skele- tal structures and the utility of developmental osteology in taxonomic studies. Among the fIrst bones to ossify are those associated with feeding, respiration, and orientation (e.g., jaws, bones of the branchial region, clei- thrum, and otoliths). The axial skeleton follows with formation of vertebrae and asso- ciated bones. Once the axial skeleton is suffIciently established, median- and pelvic- fm supports form, and fIns develop. Pre- sence, number, position, and shape of certain bones in many parts of the skeleton can have diagnostic value, even for closely related species. Use of osteological characters for identification of fIsh larvae has received little attention, but its potential value is great, particularly for confrrmation of questionable identities and for species in which external characters are diagnostically inadequate. SPECIMENS EXAMINED Cultured specimens were analyzed for each species. Developmental series for all but Utah sucker were reared by the Larval Fish Laboratory from artifIcially-fertilized eggs during 1978 through 1981. Parental stock for culture of flannelmouth sucker was collected from the Yampa River near Juniper Springs; bluehead sucker from the White River near Rio Blanco Lake; mountain suc- ker from Willow Creek, headwater tributary to the Elk River (tributary to the Yampa River) northwest of Steamboat Springs; razorback sucker from a gravel pit off the Colorado River near Clifton; and white sucker from a private pond southwest of Fort Collins. Razorback sucker larvae and juven- iles were reared also by Dexter National Fish Hatchery in 1982 from Lake Mohave stock. Utah sucker specimens were reared by the Utah Cooperative Fish and Wildlife Research Unit in 1987 from Bear Lake stock. Wild or fIeld-collected specimens of certain identity were analyzed also for all species except Utah sucker. Flannelmouth, bluehead, and white sucker larvae and juven- iles were collected during 1976 through 1979 from the Yampa River from west of Milner to the Lily Park area below Cross Mountain Canyon. Analyzed tlannelmouth and blue- head sucker larvae were collected also from the White River between Rio Blanco Lake and Spring Creek. Additional flannelmouth and bluehead sucker specimens were cursorily examined from 1977 through 1979 collections in the Colorado River between Palisade and the Colorado-Utah border and in the Gunni- son River between Whitewater and Redlands Dam. Analyzed mountain sucker specimens were collected in 1981 from Willow Creek (and Ways Gulch) in Colorado, and in 1982 through 1986 from the Provo and Spanish Fork Rivers in Utah. Mountain suckers collected in 1973 through 1982 from the Truc- kee River and Pyramid Lake, Nevada, and in 1966 and 1967 from Rocky Creek, Madison River, and Flathead Creek (all tributaries of the Missouri River) in south central Montana were cursorily examined. Partial series of razorback sucker larvae were analyzed from collections in Lake Mohave and a March 20, 1984, collection from the Salt River at Horse- shoe Bend in Arizona (the latter were reared at Dexter National Fish Hatchery and stocked a week prior to capture). Most of the collected and reared speci- mens were killed and fIxed in 10% formalin, then stored in 3% buffered formalin. Some mountain sucker specimens from Montana were preserved in ethyl or isopropyl alcohol solutions. Due to excessive dehydration and shrinkage, none of the latter specimens were analyzed for measures or size relative to developmental state. All specimens used for morphometric and skeletal analysis, and most of other speci- mens on which this study is based are main- tained in collections of the Larval Fish Labor- atory and are available for examination by other researchers. Individual specimen data (counts and measures) are stored in IBM PC- compatible computer ftles (Lotus 123). 15