352 THE FRESH-WATER MUSSEL LESS, xxvm 



Beneath the anterior end of the hinge the pallial line ends 

 in an oval mark, the anterior adductor impression (a. ad], 

 into which is inserted one of the muscles which close the 

 shell. A similar, but larger, posterior adductor impression 

 (p. ad) lies beneath the posterior end of the hinge. Two 

 smaller markings in close relation with the anterior adductor 

 impression mark the origin of the anterior retractor (a. r), 

 and of the protractor (prc) of the foot : one connected with 

 the posterior adductor impression, that of the posterior 

 retractor (p. r) muscle. 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. 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 o'f calcium carbon- 

 ate, 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 continuous structure. 



By the removal of the shell the body of the animal 

 (Fig. 92, B) is seen to be elongated from before backwards, 

 narrow from side to side, produced on each side into a 

 mantle-lobe (m), and continued ventrally into a keel-like 

 visceral mass, which passes below and in front into the 

 foot (ft). Thus each valve of the shell is in contact with 



