98 PARTS OF BIVALVE SHELLS. 



When upon a similar division, a slight difference is observed 

 in the two valves, the shell is termed subequilateral ; but 

 where the difference is well marked, it is an inequilateral bivalve 

 (Jig. 117). When the apex is bent, as is commonly the 

 case, out of the transverse plane, it is always directed more or 

 less towards the anterior part of the shell ; if such a bivalve 

 shell as the Cytherea, or Isocardia, be held before the observer, 

 with the umbones directed forward, and the hinge above, in the 

 position, in fact, in which the living animal would place itself 

 if it were creeping forwards from the observer, the right valve 

 will of course correspond with the right hand of the observer, and 

 the left with the left. 



Superior or dorsal border. 



* 



Fig. 117. LEFT VALVE OF A CYTHEREA. 



The annexed figure (117) represents the left valve of a. 

 Cytherea, the common clam belongs to this group, and would 

 answer our purpose as well. You see that the superior or 

 dorsal margin is on the side where the hinge is placed, and the 

 ventral or inferior border opposite to it; A, is the anterior 

 margin or slope ; P, the posterior margin or slope ; the umbo 

 is marked " BEAK," and s, points to the summit or apex of the 

 beak ; /, shows the situation of the Ivnule. Now, if a bivalve 

 in which the apices have a spiral twist, as in the Isocardia 

 (fig. 104), be placed in the above position, and compared with 

 the univalve shell of the Purpvra, it will be seen that the left 

 valve corresponds with the ordinary or dextral spiral univalve, 

 and the right valve to the perverse or sinistral univalve. 



When the circumference or margin of one valve fits exactly, 

 at every part, that of its fellow, it is said to be " regular" or 

 ".entire ;" but if it be notched at any part, so as not to come 



