MOLLUSCA. LAMELLIBRANCHIA 209 



Most of the lamellibranchs are free, but a few forms, 

 such as the oyster, are permanently attached by one valve, 

 which adheres firmly to a rock or some other object. In 

 some cases the right valve is fixed, in others the left. The 

 shell in these forms becomes irregular and the fixed valve 

 is larger and thicker than the free valve. Other genera 

 are attached by means of a byssus (p. 199), which often 

 passes out through a notch or sinus in the margin of one 

 or both valves. In the free forms, movement takes place 

 usually by means of the foot, but some genera (Pecten, 

 Lima) move by the rapid opening and shutting of the 

 valves. A few are capable of making borings into various 

 substances ; thus Teredo, the ship-worm, bores into wood, 

 Lithodomus and Saocicava into limestone, and Pholas into 

 various materials, such as sandstone, limestone, gneiss, 

 peat, and amber. Wood perforated by Teredo has been 

 found fossil in various formations of Eocene and Oligocene 

 age. 



The features which more especially characterise the 

 lamellibranchs as a class are : the absence of a head and of 

 organs of mastication, the bilateral symmetry, the division 

 of the mantle into two lobes, the bivalve shell and the 

 lamellar gills. Although at first sight the shell appears 

 to resemble closely that of the brachiopods, it differs 

 in several important respects : — (1) the valves are right 

 and left, instead of dorsal and ventral, (2) they are 

 generally inequilateral and equivalve, (3) teeth occur on 

 both valves, (4) a ligament is present, (5) the umbones 

 are never perforated for a peduncle, (6) the microscopic 

 structure of the shell is different. 



Various classifications of the Lamellibranchia have been, 

 from time to time, proposed. By Lamarck this class was 

 divided into the Monomyaria and the Dhnyaria, depending 



w. p. 14 



