326 



MOLLUSCA. 



While the outer surface of the shell presents various sculpture 

 markings, the inner surface is smooth and shines with the lustre 

 of mother-of-pearl. On a closer examination impressions and pits 

 become visible 011 the inner surface. A narrow line, the so-called 

 mantle or pallia! line (the line of attachment of the pallial 

 muscle of the mantle-edge to the shell), is placed near and 

 fairly parallel to the ventral edge of the shell (Fig. 251). In 

 the siphoned forms this presents posteriorly a bend directed 

 forwards and upwards (Mb] the pallial lay or indentation (Sinn- 

 palliatae). 



This curve in the attachment of the mantle-edge permits of the 

 siphons being contained within the shell when retracted. Impres- 

 sions are usually caused by the insertion of an anterior and 

 posterior adductor muscle which pass through the body transversely 



from one side to the 

 other, and are attached 

 to the inner surface of 

 the shell (Fig. 251). 



While in the equi- 

 valve mussels (Ortho- 

 conclta] the two im- 

 pressions are usually 

 of equal size, in the 

 unequi valve forrns(P/ez<- 

 roconcha} the anterior 

 adductor may be re- 



FIG. 252. A vicula semisagitta, the valves are shifted over duced, and even COm- 

 one another. M impression of the single adductor 1.1.1 i 1-1 



muscle. pletely vanish, in which 



case the posterior 



adductor has a much larger size and shifts forwards to the middle of 

 the shell. Hence the names Dimyaria and Monomyaria. 



Chemically the shell consists of carbonate of lime and an organic 

 matrix (concliyolin\ which usually has a laminated texture. In 

 addition to this laminated layer there is also a thick external 

 calcareous layer composed of large, pallisade-like prisms, which are 

 placed side by side and may be compared to the enamel of teeth 

 (Fig. 253). Finally, on the outer surface of the shell there is a 

 horny cuticle, the so-called epidermis or periostracum. 



The internal laminated layer is called the nacreous or mother-of- 

 pearl layer ; it has an iridescent lustre, and is secreted by the 

 whole surface of the mantle. The middle layer is called the 



