I 



L AMELLIBRANCHIATA. 465 



valves, such as the Oysters, habitually lie on one side, in 

 which case the valves, though really right and left, are called 

 " upper " and " lower." It is to be borne in mind also that 

 the two valves, especially in the attached Bivalves, may be 

 very unsymmetrical, one valve being much larger or deeper 

 than the other. Lastly, there are some cases in which the 

 shell becomes very nearly equilateral, the line drawn from 

 the beaks to the base dividing the shell into two almost 

 equal halves. 



Fig. 322. Left valve of Cytherea chione (after Woodward). A, Anterior margin ; B, Pos- 

 terior margin ; c, Ventral margin or base, u, Umbo ; h, Ligament ; I, Lunule ; c, Cardinal 

 tooth ; t, t, Lateral teeth ; a, Anterior adductor ; a' Posterior adductor ; p, Pallial line ; 

 s, Pallial -sinus, caused by the retractor muscles of the siphons. 



The following are the chief points to be noticed in con- 

 nection with the shell of any Lamellibranch : Each valve of 

 the shell may be regarded as essentially a hollow cone, the 

 apex of which is turned more or less to one side ; so that 

 more of the shell is situated on one side of the apex than on 

 the other. The apex of the valve is called the " umbo," or 

 "beak" (fig. 322, u\ and is almost always turned towards 

 the mouth of the animal. Consequently the side of the shell 

 towards which the umbones are turned is the "anterior" 

 side, and it is usually the shortest half of the shell. In 

 some Bivalves, however, the beaks are " reversed," and are 

 turned towards the posterior side of the shell. The longer 



VOL. I. 2 G 



