What is a genus of extinct Paleozoic brachiopods, often very abundant in the Upper Silurian. Pentamerus limestone (Geol.), a Silurian limestone composed largely of the shells of Pentamerus called?

pentamerus — A genus of extinct Paleozoic brachiopods, often very abundant in the Upper Silurian. Pentamerus limestone (Geol.), a Silurian limestone composed largely of the shells of Pentamerus. (Webster's 1913.)