MOLLUSC A. LAMELLIBRANCHIATA. 135 



(1) The umbones are generally directed anteriorly. 



(2) The lunule is always anterior to the umbones. 



(3) The external ligament is never anterior. 



(4) The pallial sinus is posterior. 



(5) When one adductor impression only is present, it 



is the posterior. 



Having found the anterior and posterior margins, the 

 shell should be placed with the dorsal margin uppermost 

 and the anterior margin pointing away from the observer, 

 then the right and left valves will be on his right and left 

 hand sides respectively. 



Most of the lamellibranchs are free, but a few forms 

 like the oyster are permanently attached by one valve. In 

 some cases it is the right valve which is fixed, in others 

 the left. In the free forms movement takes place usually 

 by means of the foot, but some genera (Pecten, Lima) 

 move by the alternate 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'inio 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 bilateral symmetry, the 

 mantle being divided into two lobes, the bivalve shell, the 

 lamellar gills, and the absence of a head and of organs of 

 mastication. Although at first sight the shell appears to 

 resemble closely that of the brachiopod, it differs in 

 several important respects, namely, (1) in the valves being 

 right and left, instead of dorsal and ventral, (2) in being 

 generally inequilateral and equivalve, (3) in the presence 



