protostracum
From ZooTerms (Dictionary of Invertebrate Zoology)
protostracum (noun; Greek protos, first; ostrakion, a shell): (Mollusca: Bivalvia) 1. In larval forms, the first formed part of a prodissoconch. 2. In oysters, the shell of the D-shaped larval stage; straight-hinge veliger.
