Abstract
An ontogenetic series of spines smaller than but very similar to the form taxon Physonemus attenuatus Davis have been found to be anterior dorsal spines of a cladodont chondrichthyan. The specimens, from the Upper Mississippian (Chesterian) Bear Gulch Limestone of Montana, are closest to the Stethacanthidae although they lack a modified dorsal fin following the spine and have prominent metapterygia in the pelvic fins. The new genus and species Damocles serratus is erected for them and they are provisionally included within the Stethacanthidae together with the referred species Damocles attenuatus (Davis).

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