2O6 BY THE DEEP SEA. 



two valves, it has some additional ones. The hinge-plates 

 are reflected back over the beaks, and a powerful muscle is 

 attached thereto to keep the ordinary valves together. Above 

 these, and to protect this muscle, are two short accessory 

 valves, and a third, which is long, and extends back over the 

 dorsal edges of the big valves. In other species of Pholas 

 these arrangements give scope for variation. 



And now it is time we gave some thought to the one-valved 

 and valveless mollusks of the shore. 



