The royal natural history . DATE SHELLS IN THE ROCK (liat. size ruins must at some time have been submerged, and that the coast has changed itslevel within historic times at several epochs. BIVAL VES. 407 Order Pseudolamellibranchiata. The gills in this order are folded, and their filaments furnished with conjunc-tive or vascular interfoliary junctions; the mantle being free all round, and thefoot small or absent. Avicididce, Prasinidce, Ostreidce, Pectinidce. Limidce,Spondylidce, and Dimyidcv are the families constituting this order. The firstfamily (Avicididce) is of importance as including
The royal natural history . DATE SHELLS IN THE ROCK (liat. size ruins must at some time have been submerged, and that the coast has changed itslevel within historic times at several epochs. BIVAL VES. 407 Order Pseudolamellibranchiata. The gills in this order are folded, and their filaments furnished with conjunc-tive or vascular interfoliary junctions; the mantle being free all round, and thefoot small or absent. Avicididce, Prasinidce, Ostreidce, Pectinidce. Limidce,Spondylidce, and Dimyidcv are the families constituting this order. The firstfamily (Avicididce) is of importance as including the pearl-oysters. The shells aremostly compressed, but vary much in outline. In the typical genus Avicula theshell is oblique with a straight hinge-line, more or less produced into wings, which. PEARL-OYSTERS (., nat. size) are sometimes long and slender. In the pearl-oysters, Meleagrina, there are, how-ever, no wings, and in the case of M. margaritifera the shells become very thickand heavy. This species is one of the principal pearl-producers, and is largelycollected by divers off the north coasts of Australia and other places, not only forthe pearls which they may contain, but also for the shells themselves, whichare valuable as mother-of-pearl. Until recently, the fishery was carried on bynative divers, but now the diving-dress is largely employed. The Ceylon pearl-oyster, M. fivcata, is much smaller than the Australian species. This fishery hasbeen carried on for over two thousand years, and the accumulation of shells is soenormous as to extend for miles several feet deep. The shells are thin and oflittle use as mother-of-pearl, consequently they are thrown away, after being 4o8 MOLLUSCS. examined for pearls. Pearl-fishing is also carried on in the Persian Gulf, theSouth
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
