endostracum
From ZooTerms (Dictionary of Invertebrate Zoology)
endostracum (noun; Greek endon, within; ostrakon, shell): 1. (Arthropoda: Chelicerata) For arachnids, see epiostracum.
2. (Arthropoda: Insecta) The endocuticle.
3. (Mollusca) The inner layer of a shell.
