Helicoprionidae is an extinct, poorly known family of bizarre holocephalids within the poorly understood order Eugeneodontida. Members of Helicoprionidae possessed a unique "tooth-whorl" on the symphysis of the lower jaw as well as pectoral fins supported by long radials. The closest living relatives of Helicoprionidae and all other eugeneodontids are the ratfishes. The anatomy of the tooth-whorl differed amongst genus and species, some possessing complete spirals (such as those of Helicoprion), others possessing halved spirals (seen in Parahelicoprion), and some with wedged half-spirals (seen in Sarcoprion). Each tooth-whorl is thought to be adapted to a different type of prey, and a different predation strategy.