periostracum


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periostracum

[‚per·ē′äs·trə·kəm]
(invertebrate zoology)
A protective layer of chitin covering the outer portion of the shell in many mollusks, especially fresh-water forms.
References in periodicals archive ?
for the first time, by SEM prints at 100X magnification which revealed microscopic accompanying organisms inserted in the hirsute periostracum, possibly in symbiosis with the host (Figure 2).
Our results clearly indicate that the "exotic" mussels with shells with slightly purplish interior and a thick black periostracum are not only morphologically distinct but also genetically distinct from Perna viridis, the Asian green mussel, and the horse mussel Modiolus modulaides (Fig.
Shell lustreless, covered with a predominantly mid-brown periostracum when fresh, with irregular, pale, flake-like markings; underlying shell mostly pale with darker brownish spiral bands either side of a pale, peri-umbilical band; sometimes also darker behind flared aperture lip.
The shell of Haliotis discus hannai Ino is polycrystalline composites of calcium carbonate and proteins and glycoproteins and consists of the periostracum, prismatic and nacreous layers with calcite in the outer prismatic layer and aragonite in the inner nacreous layer.
69) well-preserved, up to 15 mm in length but more commonly less than 10 mm, dextral, relatively thick; spire acute-conical, bearing up to 7 whorls, sutures deeply impressed; growth lines inconspicuous, worn; aperture tear-shaped (Harmen & Berg 1971); subfossils lack periostracum. Stratigraphy: Marl.
The external valve surface shows clear growth lines and the periostracum is not persistent, allowing prism cross sections to be readily discerned.
Paine (1963) also points out that the phosphatic shell of Glottidia and its thin organic periostracum decay rapidly in the sediments.
Different layers of the shell will have different proportions of calcite, aragonite, and the organic material: the hypostracum, for example, has a much higher proportion of aragonite than the subnacreous layer, which has multiple sheets of calcite separated by conchiolin proteins; the prismatic layer has crystals of both calcite and aragonite; the periostracum is composed almost entirely of organic material (Galtsoff 1964).
Bivalves are commonly articulated and typically preserved as composited external and internal molds (Pojeta 1971) coated by a black film, presumably the remains of periostracum. Calcitic brachiopods are relatively rare and represented mostly by Zygospira and Platystrophia clarksvillensis.
meretrix observed then, most of them still with the periostracum intact, suggest that the species may have died out in the system only recently.