Source : Webster's Revised Unabridged Dictionary (1913)

Rissoid \Ris"soid\, n. [NL. Rissoa, the typical genus ( fr. A.
   Risso, an Italian naturalist) + -oid.] (Zo["o]l.)
   Any one of very numerous species of small spiral gastropods
   of the genus {Rissoa}, or family {Rissoid[ae]}, found both in
   fresh and salt water.