The Collaborative International Dictionary of English v.0.48:

Rostellum \Ros*tel"lum\, n.; pl. Rostella. [L. See Rostel.]
   A small beaklike process or extension of some part; a small
   rostrum; as, the rostellum of the stigma of violets, or of
   the operculum of many mosses; the rostellum on the head of a
   tapeworm.
   [1913 Webster]