Diagnostic Features:
Medium to large in size (0.15 to 8.0 m), the body is subsylindrical, triangular, or compressed in cross sections. One or two dorsal fins may or may not be preceded by a spine or spines; an anal fin is present or absent; the caudal fin is heterocercal to almost diphycercal. Five to seven lateral gill slits and arches are present; spiracles vary from well developed to absent. The mouth is subterminal to almost terminal. The eyes lack a nictitating membrane. The cloaca is confluent with the inner margins of the pelvic fins. The rostrum is trough-shaped, and connected to the precerebral cavity by a fossa or tube. The cranium lacks suborbital shelves; the basal angle is weakly to strongly developed; the nasal capsules are not greatly elongated laterally or anteroposteriorly; the preorbital walls are complete, but the postorbital walls are incomplete; the supraorbital crests are well developed; the otic capsules are relatively long, and usually bear lateral commissures. The upper jaw (palatoquadrate) has an anterior articulation with the cranium. The pectoral girdle usually has a small laterally positioned propterygium. long metapterygium. and short metapterygial axis. From 75 to 86 living species are classified in 5 families and 3 orders of this superorder; they are largely confined to cold and deep waters. Most species are benthic or largely associated with the bottom, but a few are mesopelagic or bathypelagic.