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. A fossil echinus of the genus Cannulus; -- so called from its supposed resemblance to a cap.(Paleon.)