MOLLUSCA. LAMELLIBRANCHIATA. 



131 



is important to remember that it is always anterior to the 

 umbones. Behind the umbones there is sometimes a 

 somewhat similar, but larger area, bounded by a ridge, 

 and known as the escutcheon. 



In the interior of the valves various markings will be 

 noticed (fig. 45 B), these are produced by the union of the 

 muscles with the shell. The adductors form prominent 



FIG. 45. Cytherea chione. Eecent. A, dorsal view of the two valves. 

 B, interior of left valve. 



a, anterior border ; p, posterior ; d, dorsal ; v, ventral ; lu, 

 lunule ; u, umbone ; I, ligament ; aa, anterior adductor impression ; 

 pa, posterior adductor ; pi, pallial line ; , pallial sinus ; w, x, y, 

 cardinal teeth ; z, anterior-lateral tooth. 



oval or round, sometimes elongated, depressions (the 

 adductor impressions, aa, pa,), there are two in each valve 

 in the Dimyaria, one being anterior, the other posterior ; 

 sometimes they are of the same size, in others the anterior 

 is smaller. In the Monomyaria there is one impression 

 only which is then placed posteriorly or sometimes almost 

 central. Less important than the adductor impressions 

 are those produced by the muscles for the retraction of 

 the foot, of which there are two in each valve ; of these 



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