Some definitions from shell sites:

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Escutcheon: 

A depressed area in some bivalves, behind the beaks in one or both valves, set off from the rest of the shell by a change in colour or sculpture, and sometimes bordered by a ridge.

​Dorsal depression of the two valves behind the umbones, through which the ligament extends; its surface is smooth, unlike the rest of the shell.

​An area of the posterodorsal shell surface near a ligament that is differentiated by sculpture and frequently demarcated by a marginal ridge or furrow.

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