vin SHELL 399 



shell of muscular fibres from the edge of the mantle : the 

 streak is hence called the pallial line. Beneath the anterior 

 end of the hinge the pallial line ends in an oval mark, the 

 anterior adductor impression, into which is inserted one of the 

 muscles which close the shell. A similar, but larger, posterior 

 adductor impression lies beneath the posterior end of the 

 hinge (compare Fig. 101,0. ad, p. ad). Two smaller markings 

 close behind the anterior adductor impression, dorsal and 

 ventral respectively, mark the origin of the anterior retractor 

 and of the protractor muscle : one just anterior to the 

 posterior adductor impression, that of the posterior retractor. 

 From all these impressions faint converging lines can be 

 traced to the umbo : they mark the gradual shifting of the 

 muscles during the growth of the animal. 



The shell consists of three layers, the outer layer, as in 

 the crayfish, being uncalcified. Outside is a brown horn- 

 like layer, the periostracum, composed of conchiolin, a 

 substance allied in composition to chitin. Beneath this is 

 a prismatic layer formed of minute prisms of calcium car- 

 bonate, separated by thin layers of conchiolin ; and, lastly, 

 forming the internal part of the shell is the nacre, or 

 " mother-of-pearl," formed of alternate layers of carbonate 

 of lime and conchiolin arranged parallel to the surface. 

 The periostracum and the prismatic layer are secreted 

 from the edge of the mantle only, the pearly layer from the 

 whole of its outer surface. The hinge-ligament is continu- 

 ous with the periostracum, and is to be looked upon simply as 

 a median uncalcified portion of the shell, which is therefore, 

 in strictness, a single continuous structure. 



By the removal of the shell the body of the animal is seen 

 to be elongated from before backwards, narrow from side to 

 side, produced on either side into a mantle-lobe, and con- 

 tinued ventrally into a keel-like visceral mass, which passes 



