LAMELLIBRANCIIIATA. 



6$? 



It is to be remembered, however, that many of the Bivalves, 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 (e.g., Pectunculus) 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. 



The following are the chief points to be noticed in connection 

 with the shell of any Lamellibranch (fig. 554) : 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," and is mostly turned 

 towards the mouth of the animal. Consequently, the side of the 



Fig. 554. — Left valve of Cytherea chione. (After Woodward.) a, Anterior margin; b, 

 Posterior margin ; c, Ventral margin or base, tt, Umbo ; h, Ligament ; /, Lunule ; c, Cardinal 

 tooth ; 1 1, Lateral teeth ; a, Anterior adductor ; a , Posterior adductor ; p, Pallial line ; s, Pallial 

 sinus, caused by the retractor muscles of the siphons. 



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 half of the shell, from which 

 the umbones turn away, is called the " posterior " side, but in some 

 cases this is equal to, or even shorter than, the anterior side. The 

 side of the shell where the beaks are situated, and where the valves 

 are united to one another, is called the " dorsal " side ; and the 

 opposite margin, along which the shell opens, is called the " ventral " 



