A new elasmobranch, Thrinacoselache gracia, is described from the Bear Gulch Limestone lens (Heath Formation; Sepukhovian) of Montana, with an elongate neurocranial region, occipital region that incorporates several separate vertebral zygal elements, extremely elongate, low body, and anatomically heterocercal, but horizontally oriented tail. Teeth are borne upon the rostrum, palatoquadrate, and mandible. The palatoquadrates articulate with both palatobasal and postorbital processes, as well as with the ethmoid; the hyomandibula articulates with the posterior upper corner of the neurocranium remote from the postorbital process. The mandible is longer than the neurocranium and meets its contralateral element in an extended, but mobile symphysis. Squamation is placoid, except for a dorsal pre-otic shield of elongate denticles. Digestive cavity contents identifiable as elasmobranch and holocephalan indicate that Thrinacoselache was predominantly a piscivore. The teeth are close to isolated teeth described as Thrinacodus, and particularly to the twisted, asymmetrical teeth of T. ferox Turner rather than the symmetrical teeth of the genotype, T. nanus. Tooth asymmetry and elongated preotic scale morphology in Thrinacoselache gracia are attributed to the strong positive allometry of the rostral region relative to the remainder of the neurocranium. The implications of distinctly different growth patterns between Thrinacodus and Thrinacoselache support the removal of T. ferox and its assignment to the new genus. The Thrinacodus-complex is removed from the Phoebodontidae and assigned to the new family Thrinacodontidae. Phylogenetic analysis indicates that Thrinacoselache and Diplodoselache occupy a basal position relative to other elasmobranchs for which holomorphological information is available.