Commonly, the cleithrum is a large, well-ossified bone provided with a dorsal and a ventral arm that form an angle of about 90 or a few more degrees. The dorsal arm may end in a sharp or spine-like process to articulate with the supracleithrum. At the confluence of both arms the bone presents a blade extending postero-ventrally and that may have different development among fishes. The anterior edge of the cleithrum is usually thick and may produce a medial plate of different degrees of development.[TAO]

relationship loss: part_of scapular area (AAO:0000935)[AAO]

Paired bones of intramembranous origin. Each cleithrum extends medially from a point near the suprascapular-scapular articulation to invest the anterior margin of the suprascapula, and occasionoally it may extend to the dorsal (medial) surface. Usually, this bone is bifurcate.[AAO], Dermal bone that articulates dorsally with the supracleithrum and ventro-medially with the scapula and coracoid and mesocoracoid bone when the last bone is present. With the posttemporal and supracleithrum, the cleithrum supports the primary pectoral girdle. The cleithrum is located immediately behind the branchial cavity.[TAO]

uberon

UBERON:0004741

http://purl.obolibrary.org/obo/NCBITaxon_40674

Found in some early members of Chelonia (e.g. Triassochelys), but missing in all later forms.