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Female, head disarticulated and displaced. The fish is embedded in a mass of filamentous algae. Alternative combination: Physonemus falcatus. "The genus Physonemus was originally erected for P. arcuatus by McCoy (1848) to receive elegant forwardly-curved, well omamented Paleozoic fin spines of unknown affìnities ... The Paleozoic fin spine Physonemus falcatus St. John and Worthen 1883, from the Valmeyeran St. Louis Limestone of St. Louis, Missouri, has been found on sexually mature males of a small, highly sexually dimorphic chondrichthyan from the Chesterian Bear Gulch Limestone of Montana" (Lund 1985, p.1). Diagnosis from Lund 1985, p.3: “Small sharks, maximum known size 145 mm fork length, with forwardly-curved spines, inclined at mean angle of 14.5 degrees to the horizontal. Neither spine nor first dorsal fin present on juveniles below 124 mm length, nor on females. Males with elongate rostrum, denticles covering dorsum of rostrum, cranium and dista] portion of dorsal rod, females with short rostrum and devoid of denticles. Teeth delicate, cladodont, tooth whorls of larger, non-cladodont teeth external to jaws. Pectoral fin with five prearticular basals, a metapterygium bearing six radials, and an axis of seven elements with only one radial. Pelvic girdle supports 8—10 radials; clasper of male consisting of 6—8 axials and a three-part mixopterygium. Second dorsal fin of 12—14 well spaced radials followed by a triangular basal plate, l9—21 distal radials. Axial skeleton of 44 precaudal segments, 18 preural caudals and 16—18 ural caudals. Caudal fin deeply forked, equilobate, with no radials between dorsal and ventral lobes.” Line drawing from Lund 1985, p. 13: References: St. John, O. H. and Worthen, A. H. (1875). Descriptions of fossil fishes. Geological Survey of Illinois 6:245—488. St. John, O. H. and Worthen, A. H. (1883). Descriptions of fossil fishes; a partial revision of the Cochliodonts and Psammodonts. Geological Survey of Illinois 7:55—264. Lund, Richard (1985). The morphology of Falcatus falcatus (St. John and Worthen), a Mississippian stethacanthid chondrichthyan from the Bear Gulch Limestone of Montana, Journal of Vertebrate Paleontology, 5:1-19, DOI: 10.1080/02724634.1985.10011842.