MOLLUSCA. LAMELLIBRANCHIATA. 133 



these muscles is performed by the ligament. This 

 consists of two parts, the external (fig. 45 I), and the 

 internal (sometimes erroneously termed the cartilage.) 

 One or other may be absent. The external ligament is 

 composed of horny material ; it is placed outside the 

 shell and is either under the umbones or posterior to 

 them, it passes from one valve to the other and is 

 frequently attached to more or less prominent ridges. 

 When the valves of the shell are closed the external 

 ligament is stretched, and consequently in order to open 

 the shell, the animal has merely to relax its adductor 

 muscles. The internal ligament consists of parallel elastic 

 fibres, and is placed in grooves or sockets along the hinge- 

 line, 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 india-rubber placed 

 in the hinge-line of a door. Occasionally the ligament is 

 preserved in fossil specimens. The length of a lamelli- 

 branch shell is measured from the anterior to the 

 posterior margin (fig. 45 B, a p), the breadth or height 

 from the umbone to the ventral margin (d v), the thick- 

 ness from one valve to the other at right angles to the 

 lines of length and breadth. 



The shell is secreted by the mantle and consists of 

 two calcareous layers, the inner is the pearly or nacreous 

 layer, and consists of numerous lamellae ; the outer is the 

 prismatic layer (fig. 46), and consists of prisms placed at 

 right angles to the surface of the shell. The external 

 surface is covered by a thin green or brownish epidermis. 

 The prismatic layer is formed by the margin of the 

 mantle only, the pearly layer by the whole of the rest of 

 the mantle, and this layer gradually encroaches on the 

 former, which consequently cannot afterwards increase in 



