XXVI 



INTRODUCTION. 



seen ; the marks a indicate the position of the 

 muscular scars or impressions made on the shell by 

 the attachment of the adductor mttscles which serve to 



C'y 



a) 



draw the valves together. The number of the 

 muscles is not in all cases the same : Conchifera or 

 Bivalves have therefore been divided into two groups, 

 named respectively Monomyaria and Diniyaria. In 

 the former and smaller group the animals have only 

 one adductor muscle, which is placed in, or near, the 

 centre of the body. In the latter and much larger 

 group there are two distinct adductor muscles on the 

 right and left sides of the body. 



In addition to the impressions made in the interior 

 of the valves by these muscles, there is a groove called 

 the pallial scar, which is caused by the muscular 

 attachment of the mantle to the shell ; it is not shown 

 in the woodcut, but if a specimen of Unio tumidus be 

 examined, it will be seen that it extends from one 



