MOLLUSC A 



141 



shell substance at all having been formed above it, and much 

 less in front than behind it, hence the very unequilateral form 

 of each valve. In other genera of fresh-water mussels the 

 two valves are held together also by " hinge-teeth," of which 

 there may be three pairs. One pair, close to the umbo, 

 forms the Cardinal teeth, and two pairs, one in front and one 

 behind the umbo, are known as the Lateral teeth. 



If the shell is removed and its inner surface examined, 

 it will be found to have a pearly white, more or less 

 iridescent appearance, except round the margin. The struc- 



FiG. 90. — The Swan JVIussel. Right valve of the shell from within. 



u, Umbo; c, c', scars of muscles which close the shell ; sf, sf, scars of muscles which 

 move the shell forwards ; sh, sh', scars of muscles which move the shell backwards ; 

 pm, the scar of the " pallial " muscle which attaches the mantle to the shell. 



ture of the shell is essentially similar to that in univalve 

 molluscs, consisting of the same three layers (see p. 108), 

 but the nacreous layer is thicker here, whilst the margin 

 of the shell consists of the horny layer alone, this being 

 at first reflected round the edge of the shell as a narrow 

 flexible fold. On the inner surface of each valve are certain 

 marks which are due to the attachments of the chief muscles 

 of the body. 



The largest scars are those of the two important " ad- 

 ductor " muscles which connect the two valves of the shell to 

 the body (Fig. 90, c, c'), and which by their contraction cause 

 the closing of the shell. Two other pairs of scars (Fig. 90, 

 sf, sf, sb, sh') are those of the muscles which cause the 

 forward or backward movements respectively of the shell 



