The Collaborative International Dictionary of English v.0.48:

crotal \crotal\ n.
   any of several lichens of the genus Parmelia from which
   reddish brown or purple dyes are made.

   Syn: crottle, crottal.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

crottal \crottal\ n.
   any of several lichens of the genus Parmelia from which
   reddish brown or purple dyes are made. [Also spelled
   crottle and crotal.]

   Syn: .
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

crotal
    n 1: any of several lichens of the genus Parmelia from which
         reddish brown or purple dyes are made [syn: crottle,
         crottal, crotal]