The influence of the
periostracum on the shell structure of bivalve molluscs.
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.
Live animals have a hairy
periostracum and prey on bivalves including oysters.
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 dark, brownish-green, opaque and usually eroded, especially in the umbo area (u).
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.
