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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.]


(Zoology) zoology the thin outer organic covering of a mollusc shell
References in periodicals archive ?
The shell lacks a periostracum, but the inner shell ranges from white to purple (Allen 1977), and the exterior of the shell may be striped purplish, brown, or yellow (Hertlein 1959).
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.
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.
Its periostracum is greenish brown to brown and darkens to black with age, and its nacre is usually deep purple (Parmalee and Bogan, 1998; Williams et al.
The periostracum is thin and one of the albuminoid, having the function of segregating deleterious substance from outer environment.
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.
Nance Ferry and McKinney Island Site deposits appear to be the result of muskrat activity, based on the overall small size of the mussels, incisor marks on the periostracum (Fig.
68) well-preserved when found but extremely fragile, sinistral, thin, globose, spire obtuseconical, body whorl extremely inflated (Harmen & Berg 1971); subfossils lacking periostracum.
However, this clam is known to experience several morphological abnormalities, including warts, pustules, discoloration of the periostracum, and infectious agents such as protozoas and Rickettsia-like prokaryotes (Kent et al.
meretrix observed then, most of them still with the periostracum intact, suggest that the species may have died out in the system only recently.