THE BIVALVES 



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h, mantle lobe; and i, j, the inner and outer gill plates. To the same family 

 belong the date shells (Lithodomus} , which, as shown in the illustration, are 

 stone borers. The shells are date-like, thin, and covered with a yellowish or 

 brown periostracum. The boring is chiefly effected by the foot. The columns of 

 the temple of Serapis at Puteoli are perforated by a species of these mollusks, at a 

 point far above the present sea level, thus showing that these ruins must at some 

 time have been submerged, and that the coast has changed its level within historic 

 times at several epochs. 



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. Aviculidce, Prasinid<z> Ostreidcz, Pectinidce, Limidce, Spon- 



PEARL, OYSTERS. 



(One-half natural size.) 



dylidce, and Dimyidce are the families constituting this order. The first family 

 (Avictttidte) 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 



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