The Collaborative International Dictionary of English v.0.48:

Rudistes \Ru*dis"tes\ (r[udd]*d[i^]s"t[=e]z), n. pl. [NL., fr.
   L. rudis rough.] (Paleon.)
   An extinct order or suborder of bivalve mollusks
   characteristic of the Cretaceous period; -- called also
   Rudista. See Illust. under Hippurite.
   [1913 Webster]