148 MANUAL OF NATURAL HISTORY. 



sand by means of their long conical muscular foot, 

 as the Razor-fish, the Gapers, and the fresh-water 

 Pearl-Mussels ; some again are loosely attached to 

 sub-marine rocks by a long byssus, as the Mussels 

 and Pinnas, in which case the foot, being useless, is 

 rudimentary or obsolete. Others perforate stones, as 

 the Pholades, or wood, as the Ship-worms or Tere- 

 dines, which are often very destructive to the bot- 

 toms of vessels, and do much mischief in dockyards 

 to timber. Many Bivalves are excellent articles of 

 diet, as the Scallop and the Cockle ; nor must that 

 epicurean morsel be omitted, the much-prized Oyster ; 

 other Conchiferous shell- fish are important in a 

 commercial point of view, on account of the pearly 

 nature of their shells, and the globules of free nacre 

 they sometimes secrete, as the Pearl-Oysters. 



IV. CLASS. BIVALVES (Conchifera). 



Head indistinct ; body covered with a bilobed 

 mantle, each lobe protected by a shelly valve ; gills 

 lamellar, two on each side ; foot usually compressed 

 and keeled ; mouth with elongate fleshy lips ; valves 

 of shell united on their dorsal edges by a ligament. 

 Aquatic. 



I. SUB-CLASS. BlMUSCULAR-BlVALVES 



(Dimyaria). 



Animal with two nearly equal adductor muscles 

 for closing the valves. 



I. ORDER PHYLLOPODS (Phyllopoda). 

 Foot lamellar or elongate ; gills not produced into 



