pentamerus

from The Collaborative International Dictionary of English v.0.48

Pentamerus \Pen*tam"e*rus\, n. [NL. See {Pentamerous}.]
   (Paleon.)
   A genus of extinct Paleozoic brachiopods, often very abundant
   in the Upper Silurian.
   [1913 Webster]

   {Pentamerus limestone} (Geol.), a Silurian limestone composed
      largely of the shells of Pentamerus.
      [1913 Webster]