The Collaborative International Dictionary of English v.0.48:

Hydrocorallia \Hy`dro*co*ral"li*a\, n. pl. [NL. See Hydra, and
   Coral.] (Zool.)
   A division of Hydroidea, including those genera that secrete
   a stony coral, as Millepora and Stylaster. Two forms of
   zooids in life project from small pores in the coral and
   resemble those of other hydroids. See Millepora.
   [1913 Webster]