Dictionary of Ichthyology  

cleithrum (plural cleithra) = the principal bow-shaped bone of the pectoral girdle, dermal in origin, forming the rear margin of the gill cavity. It articulates dorsally with the supracleithrum and ventrally with the scapula and coracoid, and meets its opposite pair medially under the heart. Used in age estimation, where it is more reliable than scales in some species, e.g. Esox masquinongy.

Cyprinus carpio shoulder girdle, C = cleithrum, E = scapula, PC = coracoid, F = foramen, A = attachment of fin from Kingsley's "Textbook of Vertebrate Zoology, 1906. Photograph by Brian W. Coad.

Canadian Museum of Nature