ORDER OSTRACE^E ; OYSTER. *39/ 



the whole or the greater part of their length. This group in- 

 cludes the most aberrant forms of the Lamellibranchiate Mol- 

 lusca. 



OR.DER I. ASIPHONATA. 



SECTION A. OSTRACE^E. 







1021. The shell of the Oyster is formed of two unequal 

 valves, connected together by a hinge on which there are no 

 teeth or ridges, and which is therefore of the simplest character. 

 The shell is attached by the most convex of its valves to rocks, 

 to pieces of wood, and even to others of its own kind. This 

 attachment is formed by the exact adaptation of each layer of 

 shell, prolonged beyond the margin of the former one, to the 

 inequalities of the surface upon which it lies ; and sometimes 

 the margin is actually made to re-curve backwards, to enter some 



br 



o 687. ANATOMY or THE OYSTBR : v, one of the valves of the sht-ll ; t/, itshinfjc ; 

 m, one of the lobes of the mantle ; m', a portion of the other lobe folded back ; e, adduc- 

 tor muscle ; br, gills ; b, mouth , t, tentacula, or prolonged lips ; /, liver ; /, intestine ; 

 a. anus ; co, heart. 



furrow by which it may hold more firmly. The animal itself is 

 of very simple structure. No vestige of a foot can be seen ; and 



VOL. II. F F 



