PARTS OF BIVALVE SHELLS. 



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

 " irregular," or emarginate. 



Besides the parts mentioned, 

 we shall find in most cas-es, an- 

 terior to the apices of the beaks, 

 a depression of variable extent 

 and depth. This is the lunule 

 {fig. 118, I) I it may be cordi- 

 forna, 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 (/), and its margins 

 are named lips of the fissure 

 ( If). Behind tlie 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; R, right valve; /, lunule;/! fissure; Z/', lips of the fis- 

 sure ; 5, suture ; lig^ ligament ; ny, the nymphse. 



• /LICANIEKT-, \ 

 ' / NVMPHAEv \ \ 



Fig. 118.— CYTHEREA, 



Fig. 119. RIGHT VALVE OF A CYTHEREA. 



The most important part of the margin is that which is 



