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 ?
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.
tenebricosus studied by Bonetto (1967) and the sculptures of the periostracum are similar to those described by Bonetto and Ezcurra (1965).
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.
For most of the remaining taxa, at least part of the calcitic component of the shell is made up of calcite prisms, and where these occur they are always the external shell layer immediately below the periostracum.
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).
meretrix observed then, most of them still with the periostracum intact, suggest that the species may have died out in the system only recently.
1F); (5) polychaetes do not have an external organic tube lining similar to the periostracum of mollusks and the external organic cuticle of bryozoans (Nishi 1993; Vinn et al.
luteola 0 5 303 189 Lampsilis cardium 0 0 109 118 Total number of species 2 5 20 19 Total number of individuals 3 9 980 961 (a) Ohio Endangered (b) Ohio Threatened (c) Ohio Species of Special Concern live = collected alive dead = collected as a freshly dead shell (nacre lustrous and periostracum intact) weathered = collected as a weathered dead shell (nacre not lustrous and periostracum intact) subfossil = collected as a subfossil shell (nacre and periostracum eroded) Table 2 Distribution of mussels collected from the Symmes Creek watershed prior to 2004.
Shell small, thick with opaque, coarse periostracum Radiodiscus misionensis Hylton Scott, 1957a