PARTS OF BIVALVE SHELLS. 



90 



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- i 



terior to the apices of the beaks, 

 a depression of variable extent 

 and depth. This is the lunule 

 (fg. 118, /) : 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 (/), and its margins 

 are named lips of the fissure 

 ( If). 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 Zimfr, circum- 

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

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

 valve ; 72, right valve ; 7, lunule ; f. fissure ; //", lips of the {Js- 

 sure; , suture ; lig, ligament ; m/, the nymphae. 



Fig. 118. CVTHEREA. 



?UBCARUVk .--'OFTHEPALLIAL 



IMPKESSION 



Fig. 119. RIGHT VALVE )F A CYTHERKA. 



The most important part of tho margin is that which it 



