periostracum
American Heritage Dictionary:
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per·i·os·tra·cum
(pĕr'ē-ŏs'trə-kəm) 
n., pl., -ca (-kə).
The hard chitinous outer covering of the shell of many mollusks, especially freshwater ones, that protects the shell from the erosive action of water.
n., pl., -ca (-kə).
The hard chitinous outer covering of the shell of many mollusks, especially freshwater ones, that protects the shell from the erosive action of water.
[New Latin : PERI- + Greek ostrakon, shell.]
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American Heritage Dictionary
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
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