MOLLUSCA. LAMELLIBRANCHIA 205 



hinge-plate are absent, but one or more laminae or ridges 

 are developed at the hinge-margin, e.g. Pleuromya. (7) In 

 some genera teeth are absent ; this may be a primitive 

 character, as in Grammysia, or the result of degeneration 

 as in Ostrea and Anodonta. 



In some genera (e.g. Area) there is, between the hinge- 

 line and the umbo of each valve, a flattened triangular 

 part of the shell, known as the area (fig. 93 D, e) ; when 

 this is present the umbones of the two valves are of course 

 widely separated. The lunule and escutcheon (p. 202) 

 appear to represent the anterior and posterior parts of the 

 area. Some lamellibranchs (e.g. Pecten) have, on each side 

 of the umbo, triangular or wing-like extensions of the shell, 

 known as ears. 



In the brachiopods the valves are opened by di- 

 varicator muscles, but in the lamellibranchs the work of 

 these muscles is performed by the ligament. This con- 

 sists of two parts, the external (fig. 92, I), and the internal 

 (sometimes erroneously termed the cartilage) (fig. 93 G, I). 

 One or other may be absent. The external ligament is 

 composed of horny material ; it is placed at the hinge- 

 margin, usually posterior to the umbones, and is frequently 

 attached to more or less prominent ridges; in some genera 

 (Pectunculus) the external ligament extends both in front 

 of and behind the umbones. The internal ligament 

 consists of parallel elastic fibres, and is placed in grooves 

 or sockets along the hinge, so that when the valves are 

 closed it is compressed, and, being elastic, tends to force 

 the valves apart — its action is similar to that of a 

 piece of indiarubber placed in the hinge-line of a door. 

 The external ligament acts like a C-spring, and is bent 

 when the valves are closed. Consequently, in order to 

 open the shell, the animal has merely to relax its adductor 



