The Collaborative International Dictionary of English v.0.48:

Muscarin \Mus*ca"rin\, n. (Physiol. Chem.)
   A solid crystalline substance, C5H13NO2, found in the
   toadstool (Agaricus muscarius), and in putrid fish. It is a
   typical ptomaine, and a violent poison.
   [1913 Webster]