clithrum

from The Century Dictionary.

• noun In certain Pelecypoda, a modification of the hinge-teeth to form one or more erect processes permitting only the vertical motion of the smaller valve, as in the extinct Rudistæ. In the genus Hippurites this structure is extravagantly developed.
• noun In anatomy, one of a pair of bones found in the shoulder-girdle of the stegocephalous amphibians and of some theromorphous reptiles.