Fadenia had five well-exposed gill slits, possibly with a vestigial sixth gill. There is no evidence of the specialized gill basket and fleshy operculum present in living chimaeroids. Based on the proportional size of caseodontoid tooth whorls, Lebedev suggested that Helicoprion individuals with tooth whorls reaching in diameter could reach in length, rivaling the size of modern basking sharks. The largest known Helicoprion tooth whorl, specimen IMNH 49382 representing an unknown species, reached in diameter and in crown height, which would have belonged to an individual over in length. Tooth whorls Almost all Helicoprion specimens are known solely from "tooth whorls", which consist of dozens of enameloid covered teeth embedded within a common logarithmic spiral-shaped root. The youngest and first tooth at the center of the spiral, referred to as the "juvenile tooth arch", is hooked, but all other teeth are generally triangular in shape, laterally compressed and often serrated. Tooth size increases away from the center of the spiral (abaxial), with the largest teeth possibly exceeding in length. The lower part of the teeth form projections that are shingled below the crown of the previous tooth. The lowest portion of the root below the enameloid tooth projections is referred to as the "shaft", and lies on cartilage that encapsulates the previous revolutions of the whorl. In a complete tooth whorl, the outermost part of the spiral terminates with an extended root that lacks the middle and upper portions of the tooth crown. Cartilaginous skull Helicoprion specimens preserving more than tooth whorls are very rare. The best-preserved specimen of Helicoprion is IMNH 37899 (also known as "Idaho 4"), referred to Helicoprion davisii. It was found in Idaho in 1950 and was originally described in 1966 by Svend Erik Bendix-Almgreen. A 2013 redescription by Tapanila and colleagues was accompanied by CT scanning, in order to reveal the cartilaginous remains in more detail. CT scanning revealed a nearly complete jaw apparatus, articulated in a closed position with three-dimensional preservation. Alongside the tooth whorl, the specimen preserves a palatoquadrate (forming the upper jaw), Meckel's cartilage (forming the lower jaw), and a robust labial cartilage bracing the tooth whorl. All of these structures are composed of prismatic calcified cartilage, as with modern chondrichthyans. The specimen did not preserve a chondrocranium, the cartilaginous structure which would have housed the brain and sensory organs. The jaws are extensively laterally compressed (narrow) compared to living chondrichthyans, though