FORMATION OF SHELLS. 247 



plished than the same part again shrinks, and 

 the mantle retires a little way within the shell, 

 still continuing to deposit calcareous layers, 

 which give greater thickness to the adjacent 

 part of the shell : and at the same time narrow 

 its aperture, and materially alter its general 

 shape and aspect. Thus it happens that the 

 shells of the young and of the old individuals of 

 the same species are very different, and would 

 not be recognised as belonging to the same tribe 

 of mollusca. This is remarkably the case with 

 the shell of the Cyprcea, or Cowrie, which in the 

 early stage of its growth, (Fig. 112) has the 

 ordinary form of an oblong turbinated shell : 

 but from the process j List described taking place 

 at a certain period, the mouth of the shell (as 

 shown in Fig. 113), becomes exceedingly nar- 

 row, and the edges of the aperture are marked 

 by indentations, moulded on corresponding pro- 

 cesses of the mantle.* But in this instance the 

 change does not stop here ; for both edges of the 

 mantle next take a wider expansion, turning 

 over the outer surface of the shell, and passing 

 on till they meet at the upper convex part, or 

 back of the shell, forming what has been termed 

 the dorsal line. They deposit, as they proceed, 



* Similar changes occur in the shells of the Ovula (spindles), 

 Erato (tear-shells), and Margiriclla, (dates). Gray, Phil. 

 Trans, for 1833. 



