lO-t MUSSELS, CLAMS, OYSl'KHS, SNAILS, SQUIDS 



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

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

 is made up of three laj'ers. The outer layer is a horny, 



greenish epidermis. 



Umbo 

 beah^ 



Hinge ligament 

 I)o7'sal margin 



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 l^een removed from the shell and all parts care- 

 fully scraped away. 



Anterior 

 end 



,^ ^ , Posterior 

 Ventral_ end 



margin 

 Lines of growth 



Flo, .53. —Left vaKi' of a elaiii shell. 



Lateral Posterior 

 Hinge nu'il.c retractor 

 tooth tooth muscle 



Posterior 

 udclmior 

 m iiscle 



each valve will pre- 

 sent the following 

 features: at the an- 

 terior end of the 

 shell the scar of the 

 anterior adductor mus- 

 cle will be seen; at 

 tlie postei'ior end, in 

 a siiiiilai' position, 

 is tlic scar of the 

 large poslerior ad- 

 diiclor niuKcJe; in close connection with the anterior ad- 

 ductoi- muscle, ;i.re two smaller scars, the upper one, the 

 ocar of flu; nulcrior retractor foot muscle, antl the lower, 



Fic. 



-Riclil \-al\'e of a clam .shell. 



