202 MOLLUSCA. LAMELLIBRANCHIA 



in size, and each valve is generally inequilateral. But in 

 some {e.g. Pectunculus) the shell is nearly equilateral, and in 

 others (e.g. Ostrea) it is inequivalve. Each valve may be 

 regarded as a greatly depressed hollow cone, the apex of 

 which forms the umbo (fig. 92 A, u) ; these umbones are 

 sometimes straight {e.g. Pecten), but generally curved to- 

 wards the anterior margin ; in a few genera (e.g. Nucula, 

 Trigonia, Exogyra) they are directed posteriorly ; in 

 Diceras they are spiral. Sometimes there is in front of the 

 umbones, and bounded by a groove, an oval depressed area 

 (lu), half being on each valve; this is termed the lunule. 

 Behind the umbones there is sometimes a somewhat 

 similar, but larger area, known as the escutcheon. 



In the interior of the valves various markings, 

 produced by the union of the muscles with the shell, may 

 be noticed (fig. 92 B). The adductors form oval, round, 

 or sometimes elongated depressions (the adductor impres- 

 sions, act, } m )> i n the Dimyaria there are two in each 

 valve, one being near the anterior border, the other near 

 the posterior; in the Monomyaria the single adductor 

 impression is usually near the middle of the valve. When, 

 as in the genus My a, the two muscles are placed at equal 

 distances from the hinge-margin, they are of nearly the 

 same size, since on account of their position they are 

 equally efficient in closing the valves; but in forms like 

 Mytilus, where the shell is very inequilateral and the 

 anterior muscle is close to the umbo but the posterior at 

 a considerable distance from it, the latter is much larger 

 than the former, since it is placed in a more advantageous 

 position for closing the valves. For the same reason the 

 single muscle of the Monomyaria is attached near the 

 centre of the valves. Less important than the adductor 

 impressions are those produced by the muscles for the 



