From ZooTerms (Dictionary of Invertebrate Zoology)
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epiostracum (noun; Greek epi, upon; ostrakon, shell): (Arthropoda: Chelicerata) 1. A thin elastic, colorless layer of the cuticle of arachnids which overlies the ectostracum layer; further divided into two layers: inner, dense and proteinaceous, and outer, cuticulin. 2. The upper layer of cuticle of arachnids.