IX THE LIGAMENT AND HINGE 2/1 



always single) is nearer to the posterior than to the anterior 

 side. In a similar way the pallial impression, which only forms 

 a sinus on the posterior side, furnishes a guide to the valves of 

 Dono.x, in which the beaks point backward, and of Tellina, in 

 wiiich the beaks are frequently central. 



In the fixed inequi valves (e.g. Chama) it is sometimes the 

 right, sometimes the left valve which is undermost, but the 

 fixed valve, whether right or left, is always deep, and the free 

 valve fiat. Ostrca and Aywmia are always fixed by the left 

 valve. 



The IvMule is a well-marked area in front of and close to the 

 umbones, usually more or less heart-shaped, and 

 limited by a ridge. Generally it is shallow, but 

 sometimes, as in Dosinia, Opis, and some Cardium, 

 deeply impressed. A corresponding area heliiml 

 the umbones, enclosing the ligament, is called 

 the escutcheon (Fig. 186), but it seldom occurs. 



The ligament is a more or less elastic band, 

 which unites the two valves along a line adjacent 

 to the umbones. As a rule, the greater part of ^- 



the ligament is external to the shell, but it may Fig. 186. — Venus 



, .•! •. 1 Tj- 1 1 n suhrosirata Lam.: 



lie entirely internal. It is placed, normally, gg^ escutcheon • 

 hehincl the umbones, but in a few cases, when ^'' ligament ; iv, 



11- T- 1 /J -n 7N lunule ; u, v., 



the hinge line is very long {Arcn, Fectunculiis), umbones. 



it extends in front of them as well. The edges 



of the valves, when the ligament is mainly external, are more or 



less excavated for its reception. When internal it is generally 



contained in a groove or spoon-shaped pit, known as the fossette 



(compare Fig. 187). 



The ligament consists of two distinct parts, which iiuiy occur 

 together or separately, tlie external, or ligament proper, and the 

 internal, or cartilage. Only the external portion can be seen 

 when the ^'alves are closed. As a rule, the two portions are 

 intimately connected with one another, the ligament folding over 

 the cartilage, but in some cases, e.g. 3Iya, Mactra, where the 

 cartilage is lodged witliiii ilie hinge, tliey are completely discon- 

 nected (Fig. 187). 



In Fecten the external liuainent is very thin, and runs alonff 

 the dorsal margin, while ilie internal ligament is large, solid, 

 and situated in a sliallow ])it. In Perna, wliere the hinge is 



