Helicoprionidae (sometimes referred to as Agassizodontidae)[1] is an extinct family of holocephalans within the order Eugeneodontida. Members of the Helicoprionidae possessed a "whorl" of tooth crowns connected by a single root along the midline of the lower jaw.[2] While historically considered elasmobranchs related sharks and rays, the closest living relatives of the Helicoprionidae and all other eugeneodonts are now thought to be the ratfishes.[3] The anatomy of the tooth-whorls vary between taxa, with some possessing highly specialized, coiling spirals (such as those of the namesake genus Helicoprion), while others such as Sarcoprion and Parahelicoprion possessed shorter whorls.