The Collaborative International Dictionary of English v.0.48:

Mytilus \Myt"i*lus\, n. [L., a sea mussel, Gr. ?.] (Zool.)
   A genus of marine bivalve shells, including the common
   mussel. See Illust. under Byssus.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Byssus \Bys"sus\, n.; pl. E. Byssuses; L. Byssi.[L. byssus
   fine flax, fine linen or cotton, Gr. by`ssos .]
   [1913 Webster]
   1. A cloth of exceedingly fine texture, used by the ancients.
      It is disputed whether it was of cotton, linen, or silk.
      [Written also byss and byssin.]
      [1913 Webster]

   2. (Zool.) A tuft of long, tough filaments which are formed
      in a groove of the foot, and issue from between the valves
      of certain bivalve mollusks, as the Pinna and Mytilus,
      by which they attach themselves to rocks, etc.
      [1913 Webster]

   3. (Bot.) An obsolete name for certain fungi composed of
      slender threads.
      [1913 Webster]

   4. Asbestus.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Mytilus
    n 1: type genus of the family Mytilidae: smooth-shelled marine
         mussels [syn: Mytilus, genus Mytilus]