The Collaborative International Dictionary of English v.0.48:

Castoridae \Castoridae\ n.
   a natural family comprising the beavers.

   Syn: family Castoridae.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Castoridae
    n 1: beavers [syn: Castoridae, family Castoridae]