vm SHELL 399 



distance from it, is a delicate streak caused by the 

 insertion on to the 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, 

 on to which is inserted one of the muscles which close 

 the shell. A similar but larger posterior adductor impres- 

 sion lies. beneath the posterior end of the hinge (compare 

 Fig. ioi, a. ad, p. ad}. Two smaller markings close 

 behind the anterior adductor impression, dorsal and 

 ventral respectively, mark the insertion 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 carbonate, 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 continuous 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 con- 

 tinuous 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 continued ventrally into a keel-like visceral 



