The Collaborative International Dictionary of English v.0.48:

Arcellidae \Arcellidae\ prop. n.
   A natural family of soil and freshwater protozoa;
   cosmopolitan.

   Syn: family Arcellidae.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Arcellidae
    n 1: soil and freshwater protozoa; cosmopolitan in distribution
         [syn: Arcellidae, family Arcellidae]