What is "spongia dura"

Hardhead \Hard"head`\ (-h[e^]d`), n.

1. Clash or collision of heads in contest.
   --Dryden.

2. (Zo["o]l.)

   1. The menhaden. See Menhaden. [Local, U. S.]

   2. Block's gurnard ( Trigla gurnardus) of Europe.

   3. A California salmon; the steelhead.

   4. The gray whale. See Gray whale, under Gray.

   5. A coarse American commercial sponge ( Spongia dura).