PARTS OF BIVALVE SHELLS. 



in contact with the corresponding part of the opposite valve, it is 

 " irregular," or emarginate. 



Besides the parts mentioned, A 



we shall find in most cases, an- 

 terior to the apices of the beaks, 

 a depression of variable extent 

 and depth. This is the lunule 

 (fig. 118, 1) : it may be cordi- 

 form, or crescentic, lanceolate 

 (oblong, and gradually tapering 

 towards each extremity), oval, 

 deep, superficial, &c. Behind 

 the beaks is another depression, 

 longer and narrower than the 

 lunule, and which is called the 

 fissure (f), and its margins 

 are named lips of the fissure 

 ( I/). Behind the fissure there is 

 sometimes a small depression 

 called the suture (s). The 

 general more or less convex 

 surface of each valve is called 

 the "venter," or belly, which 

 terminates in the limb, circum- 

 ference or margin. In the figure (118), the letters designate 

 parts, as follows : A, anterior slope ; P, posterior slope ; L, left 

 valve ; JR, right valve ; Z, lunule ; /. fissure ; //*, lips of the fis- 

 sure ,* s, suture ; lig, ligament ; ny, the nymphse. 



ny 



Fig. 118. CYTHEREA. 



/LIGAMENT-. \ 

 ' .MWHAEv \ \ 



gCARUTV .'OF THE PALLIAL 

 IMPRESSION 



Fig. 119. RIGHT VALVE OF A CYTHEREA. 



The most important part of the margin is that which is 



