Hypertext Webster Gateway: "Periostracum"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Periostracum \Per`i*os"tra*cum\, n.; pl. {Periostraca}. [NL.,
fr. Gr. ? around + ? shell of a testacean.] (Zo["o]l.)
A chitinous membrane covering the exterior of many shells; --
called also {epidermis}.
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