MOLLUSCA : LAMELLIBRANCHIATA. 321 



each. The shell in the Brachiopoda, as we have seen, is rarely 

 or never quite equivalve, and always has its two sides equally 

 developed (equilateral) ; whilst the valves are placed antero- 

 posteriorly as regards the animal, one in front and one behind, 

 so that they are "dorsal" and "ventral." In the Lamellibran- 

 chiata, on the other hand, the two valves are usually of nearly 

 equal size (equivalve), and are more developed on one side 

 than on the other (inequilateral) ; whilst their position as re- 

 gards the animal is always lateral, so that they are properly 

 termed "right" and "left" valves, instead of "ventral" and 

 " dorsal." 



It is to be remembered, however, that many of the Bivalves, 

 such as the Oysters, habitually lie on one side, in which case 



Fig. 129. Left valve of Cytherea chione. (After Woodward.) A, Anterior margin ; 

 B, Posterior margin ; C, Ventral margin or base ; u Umbo ; k Ligament; / Lunule ; 

 c Cardinal tooth ; / t Lateral teeth ; a Anterior adductor ; a Posterior adductor ; 

 / Pallial line ; s Pallial sinus, caused by the retractor muscles of the siphons. 



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, maybe very imsymmetrical, 

 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. 



The following are the chief points to be noticed in connection 

 with the shell of any Lamellibranch (fig. 129) : 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. 



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