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 the 

 foot small or absent. Avicididce, Prasinidce, Ostreidce, Pectinidce. Limidce, 

 Spondylidce, and Dimyidcv are the families constituting this order. The first 

 family (Avicididce) is of importance as including the pearl-oysters. The shells are 

 mostly compressed, but vary much in outline. In the typical genus Avicula the 

 shell 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 thick 

 and heavy. This species is one of the principal pearl-producers, and is largely 

 collected by divers off the north coasts of Australia and other places, not only for 

 the pearls which they may contain, but also for the shells themselves, which 

 are valuable as mother-of-pearl. Until recently, the fishery was carried on by 

 native divers, but now the diving-dress is largely employed. The Ceylon pearl- 

 oyster, M . fivcata, is much smaller than the Australian species. This fishery has 

 been carried on for over two thousand years, and the accumulation of shells is so 

 enormous as to extend for miles several feet deep. The shells are thin and of 

 little use as mother-of-pearl, consequently they are thrown away, after being 



