FORMATION OF SHELLS. 229 



The shells of Oysters and other acephalous mol- 

 lusca which adhere to rocks, are often moulded, 

 durmg their growth, to the surfaces to which they 

 are applied. The mantle, being exceedingly flexi- 

 ble, accommodates itself to all the inequalities it 

 meets with, and depositing each successive layer 

 of shell equally on every part, the figure of the 

 surface is assumed, not only by the valve in contact 

 with it, but also by the other valve, which is formed 

 by the opposite surface of the mantle,* and which 

 during its formation was immediately superposed 

 on the thin edge of the other valve, while it was 

 deflected by the irregular surface on which it grew. 



As the enlargement of the shell proceeds, it is 

 necessary that the muscle, which closes the valves, 

 and is attached to their inner surface, should be 

 gradually removed to a greater distance from the 

 hinge, so that it may preserve its relative situation 

 with regard to the whole shell, and retain undimi- 

 nished its power of acting upon the valves. For 

 this purpose its adhesions are gradually transferred, 

 by some unknown process, along the surface of the 

 valves; and the jjrogress of the removal may gene- 

 rally be distinctly traced by the marks which are 

 left in the shell at the places before occupied by 

 the attachments of the muscular fibres. The same 

 process takes place when there are two or three 

 muscles instead of one. 



A few genera of MoUusca, such as the Pholas, 

 have, in addition to the two principal valves, small 

 supplementary pieces of shell. They have been 

 accordingly comprised in the order of MiiltivalveSy 



* Defrance, Annales des Sciences Naturelles, ii. 16. 



