The Collaborative International Dictionary of English v.0.48:

Rissoid \Ris"soid\, n. [NL. Rissoa, the typical genus ( fr. A.
   Risso, an Italian naturalist) + -oid.] (Zool.)
   Any one of very numerous species of small spiral gastropods
   of the genus Rissoa, or family Rissoidae, found both in
   fresh and salt water.
   [1913 Webster]