"Shell of a Bivalve Mollusk, inner surface. ad.a., depression showing the attachment of the anterior adductor muscle; ad.p., posterior adductor muscle; h, hinge with teeth; l, attachments of the ligaments; p, pallial line, marking the attachment of the mantle muscles; s, the pallial sinus, marking the attachment of the retractor muscles of the siphon; u, umbo or beak." — Galloway

Bivalve Mollusk

"Shell of a Bivalve Mollusk, inner surface. ad.a., depression showing the attachment of the anterior…

"The following cut represents a Roman clypeus, from the column of Trajan. The projection in the centre was called the <em>umbo</em>, or boss, and sometimes a spike or other prominent excrescence, was placed upon this." &mdash; Anthon, 1891

Roman clypeus

"The following cut represents a Roman clypeus, from the column of Trajan. The projection in the centre…

"The fresh-water mussel...the inside of the shell (left valve)...u., The umbo; L., the ligament; c.t., lateral teeth; a.a., anterior adductor mark; a.r., mark of protractor of the foot; p.l., pallial line; p.a., posterior adductor mark; p.r., mark of posterior retractor of the foot; A., anterior (the blunter end); P., posterior." -Thomson, 1916

Freshwater Mussel

"The fresh-water mussel...the inside of the shell (left valve)...u., The umbo; L., the ligament; c.t.,…