hypostracum
From ZooTerms (Dictionary of Invertebrate Zoology)
hypostracum (noun; Greek hypo, under; ostrakon, shell): 1. (Arthropoda: Chelicerata) In Acari, the inner cuticular layer, usually pigmented with basic dyes.
2. (Mollusca: Bivalvia) a. Inner layer of shell wall laid down by the mantle. b. That part of the shell secreted at muscle attachments
See also: myostracum. 3. (Mollusca: Polyplacophora) In chitons, the lowest ventral calcareous layer of a valve.
