Microcystidae

Diagnosis (based on studies by Baker, 1938; Hyman & Ponder, 2010)

Shell with continuous growth and a simple, sharp peristome, rarely reduced; umbilicus closed or narrowly open; shell sculpture consisting of at most microscopic spiral grooves (most taxa) or prominent radial ribs (Fanulena Iredale, 1945 and Pittoconcha Preston, 1913). Animal with caudal apparatus formed from curled-up sole [Muratov’s (1999) helicarionid type]. Kidney unilobed; mantle with no visible minor blood vessels. Genital system ovoviviparous; oviduct non-glandular; bursa copulatrix usually very short, inserted on penis; stimulator absent; muscle fibres connecting proximal end of penial tunica to epiphallus absent; epiphallic retractor caecum and flagellum absent in most taxa. Spermatophore simple, small, peanut-shaped, soft-walled; attached to inner wall of penis. Radula with tricuspidate central tooth; lateral and marginal tooth fields distinguishable; lateral teeth usually tricuspidate (endocone absent in some taxa); marginal teeth usually multicuspidate. Penial nerve from right pedal ganglion.