104 MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 



made to the ventral edges of the umbo. The shell, which 

 is a calcareous secretion of the outer layer of the mantle, 

 is made up of three layers. The outer layer is a horny, 



greenish epidermis. 



Umbo 

 beaku 



Hinge ligament 

 Dorsal' '-ma/rqin 



Anterior 

 end 



The inner layer con- 

 sists of thin, flat leaves 

 of pearl, the fine sinu- 

 ous edges of which 

 produce an iridescent 

 effect. The dark mid- 

 dle layer consists of 

 many-sided prisms 

 placed at right angles 

 to the surface of the 

 shell. 



Internal features of the shell. — When the body of the 

 mussel has been removed from the shell and all parts care- 

 fully fecraped away, 

 each valve will pre- 



vstenor 

 end 



Ventral 

 y margin 

 Lines qf growth 



Fig. 53. — Left valve of a clam shell. 



Lateral 

 Hinge hinge 

 tooth tooth 



Posterior 

 retractor 



muscle 



Posterior 

 adductor 

 muscle 



sent the following 

 features: at the an- 

 terior end of the 

 shell the scar of the 

 anterior adductor mus- 

 cle will be seen; at 

 the posterior end, in 

 a similar position, 

 is the scar of the 

 large posterior ad- 

 ductor muscle; in close connection with the anterior ad- 

 ductor muscle, are two smaller scars, the upper one, the 

 ocar of the anterior retractor foot muscle, and the lower, 



Fig. 54. — Right valve of a clam shell. 



