256 THE MECHANICAL FUNCTIONS. 



ceediiigly flexible, 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 irregu- 

 lar surface on which it grew. As the enlargement 

 of the shell j^roceeds, it was necessary that the 

 muscle, w hich closes the valves, and is attached 

 to their inner surface, should be gradually re- 

 moved to a greater distance from the hinge, so 

 that it may preserve its relative situation with 

 regard to the whole shell, and retain undiminished 

 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 progress of the removal may 

 generally 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 Mollusca, such as the Pholas, 

 have, in addition to the two principal valves, 

 small supplementary pieces of shell. They have 



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



11 



