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n. pl. per·i·os·tra·ca (-kə)
The tough horny or fibrous outer covering of the shell of many mollusks and brachiopods.

[New Latin : peri- + Greek ostrakon, shell; see ost- in Indo-European roots.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(Zoology) zoology the thin outer organic covering of a mollusc shell
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
References in periodicals archive ?
Radiodiscus sp.) with a hirsute periostracum. The specimen discussed here (approximately 2 mm.
The surface of the clams had a chalky appearance and a deshicent olive brown periostracum was present mostly near the edges (Fig.
Yan, "Comparison of dry ashing, wet ashing and microwave digestion for determination of trace elements in periostracum serpentis and periostracum cicadae by ICP-AES," Journal of the Chilean Chemical Society, vol.
All mussels collected were identified and counted as either living, freshly dead (soft tissue present, nacre lustrous, and no erosion of the shell), weathered (dead and noticeable internal/ external erosion of shell with possible staining), or subfossil (little to no periostracum remaining with the internal/external shell often faded to white).
These mussels, with a thick black periostracum, were first found in Manila Bay near an international shipping port, and later in 2014 appeared in western Tambac Bay (approximately 16.28[degrees] N, 119.9[degrees] E).
Shell material was classified as fresh-dead (periostracum present, nacre pearly, and soft tissue may be present) or relict (periostracum eroded, nacre faded, shell chalky) based on condition of the best shell found.
In most cases, only the exoskeletons of bivalves and gastropods are depicted, with emphasis on the periostracum (the outermost layer of the shell), growth lines, and umbo.
The ground colour ranges from a pale yellowish white to a deeper pinkish or orangebrown; in fresh specimens this is overlain by a fine, irregular whitish mottling, which is in turn overlain by a thin, rather glossy, corneous periostracum. However, the latter is commonly eroded in parts, after which the whitish mottling is quickly worn off and the shell becomes more uniformly coloured.
Occasionally the color pattern of the shell is striking but usually subdued and may be hidden under the thick periostracum. Shells are more or less fusiform or ovate, with orange, brown patches on surface.