Helicarionidae Bourguignat, 1877
Diagnosis. Shell present, complete or reduced, 5-35mm in diameter; usually thin-walled, glossy; spiral grooves present on protoconch and teleoconch. Mantle with accessory lobes lying over body and shell lappets of variable size lying over shell. Sole of foot tripartite; caudal apparatus present, formed from curled up sole. Kidney unilobed; minor venation on roof of mantle cavity absent or present; mantle gland absent. Genital system oviparous; oviduct glandular. Bursa copulatrix variable in length; inserted on vagina or, if vagina absent, at junction of free oviduct and penis. Stimulator absent. Epiphallus enters penis through simple pore, fleshy lips or verge; interior of penis variable. Penial tunica present, open at proximal end, attached by muscle fibres to epiphallus. Epiphallic retractor caecum absent or present; epiphallic flagellum absent or present; where present, flagellum contains an axial filament. Spermatophore a soft capsule with hard-walled tail-pipe.
Remarks. Belongs to Helicarionoidea along with Ariophantidae and Urocyclidae; a superfamily unified by the presence of a flagellum with an axial filament, an epiphallic caecum, and mantle lobes (Hausdorf, 1998; Hyman and Ponder, 2010). However, flagellum and caecum are absent, presumably secondarily lost, in some members of all three families (e.g. Tarocystis, Sheaia, Levidens; Hausdorf, 1998; Hyman and Ponder, 2010). The boundaries of the three families are poorly defined; one character, presence of a proximally open penial tunica, distinguishes Helicarionidae from the other two families (Hausdorf, 1998; Hyman and Ponder, 2010).