IX THE LIGAMENT AND HINGE 2/1 



cases 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 

 Donax^ in which the beaks point backward, and of Tellina^ in 

 which the beaks are frequently central. 



In the fixed, inequivalves (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 

 flat. Ostrea and Anomia are always fixed by the left valve. 



The lunule 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 Car- 

 dium^ deeply impressed. A corresponding area 

 hehind 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 Fig. 186. — Venus 

 the ligament is external to the shell, but it may g^^ escutcheon; 

 be entirely internal. It is placed, normally, ^^^ ligament; lu, 

 hehind the umbones, but in a few cases, when umbones. 

 the hinge line is very long (Arca^ Pectunculus)^ 

 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 may occur 

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

 internal, or caj-tilage. Only the external portion can be seen 

 when the valves 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. Mya^ Maetra^ where 

 the cartilage is lodged within the hinge, they are completely 

 disconnected (Fig. 187). 



In Pecten the external ligament is very thin, and runs along 

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

 and situated in a shallow pit. In Perna^ where the hinge is 



