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Reference.org
Prodissoconch
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<p>A prodissoconch (meaning first or earliest or original shell) is an <a href="/facts/Embryo/9QHnLVLN">embryonic</a> or <a href="/facts/Larva/GKo2FIew">larval</a> shell which is present in the larva of a <a href="/facts/Bivalvia/IQDxHVc8">bivalve</a> <a href="/facts/Mollusk/1elgknsQ">mollusk</a> (clams, scallops, oysters, etc.). (The <a href="/facts/Homology_(biology)/muctKlyT">homologous</a> structure in <a href="/facts/Gastropod/rYPqm3PT">gastropods</a> (snails) is called the <a href="/facts/Protoconch/QgFUYQtf">protoconch</a>.) 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. </p><p>The structure of the prodissoconch has been widely used as a discriminating feature in bivalve <a href="/facts/Systematics/fagPXIkh">systematics</a>. </p>