n.1.(Zool.) Any one of very numerous species of small spiral gastropods of the genus Rissoa, or family Rissoidæ, found both in fresh and salt water.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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Two of the Truncatella species have an accessory sperm storage organ [ILLUSTRATION FOR FIGURE 5C-D OMITTED]; it resembles the upper oviduct gland in rissoids, which in some taxa has sperm-storing lobes (Ponder 1985a).