Viviparus

a genus of freshwater gastropod mollusks of the subclass Prosobranchia. The opening of the shell is covered by an operculum that permits Viviparus to survive unfavorable conditions. The young develop in the body of the mother and emerge fully formed (hence the name). Viviparus is widely distributed in stagnant bodies of water and in flowing streams. There are several species in the USSR; the most common is V. viviparus (shell up to 3.5 cm high).