The Collaborative International Dictionary of English v.0.48:

Siphonostomatous \Si`pho*no*stom"a*tous\, a. (Zool.)
   (a) Having the front edge of the aperture of the shell
       prolonged in the shape of a channel for the protection of
       the siphon; -- said of certain gastropods.
   (b) Pertaining to the Siphonostomata.
       [1913 Webster]