D defs.my
Entry 1 sense Webster, 1913

Capstone

/kăp'-stōnˌ/ · Cap·stone · IPA /ˈkæpˌstoʊn/
01 n. A fossil echinus of the genus Cannulus; -- so called from its supposed resemblance to a cap.
  1. 1.
    A fossil echinus of the genus Cannulus; -- so called from its supposed resemblance to a cap.(Paleon.)