Wikipedia
Prodissoconch
A prodissoconch (meaning first or earliest or original shell) is an embryonic or larval shell which is present in the larva of a bivalve mollusk (clams, scallops, oysters, etc). (The homologous structure in gastropods (snails) is called the protoconch.) The prodissoconch is often but not always smooth, and has no growth lines. It is sometimes still present and visible in the adult shell, if there has been no erosion of the shell in that area.
The structure of the prodissoconch has been widely used as a discriminating feature in bivalve systematics.