Also found in: Medical, Encyclopedia, Wikipedia.


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 ?
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.
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).
Paine (1963) also points out that the phosphatic shell of Glottidia and its thin organic periostracum decay rapidly in the sediments.
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.
Occasionally the color pattern of the shell is striking but usually subdued and may be hidden under the thick periostracum.
The periostracum is thin and one of the albuminoid, having the function of segregating deleterious substance from outer environment.
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.