THE OYSTER. 23 



hours it forms a tough leathery membrane fastening 

 the glass cover to the inside of the shell. At about 

 this time the invisible particles of lime begin to aggre- 

 gate and to form little flat crystals, hexagonal in out- 

 line, and about g-^F ^^ ^^ 'wxoX'i long. The crystals 

 grow and unite into little bundles or groups, and new 

 ones appear between the old ones, until, at the end of 

 six days, the film has completely lost its leathery char- 

 acter and has become stony, from the great amount of 

 lime present in it. In three or four weeks the glass 

 cover is completely built into the shell and can no 

 longer be seen, and its place is only to be traced by 

 its form, which is perfectly preserved upon the inner 

 surface of the shell. When broken out it is found to 

 be coated with a thick plate of white shell, which is 

 beautifully smooth and pearly upon the side nearest 

 the glass. 



Microscopic examination of this plate shows that it 

 is made up of an immense number of minute crystals, 

 packed and crowded together into a solid mass, with- 

 out any regular arrangement. These observations 

 show that the new layers are thrown off in the form 

 of a gummy excretion from the mantle, with the lime 

 in solution, and that the particles unite with each other 

 and form crystals while the gum is hardening. 



The oyster obtains the lime for its shell from the 

 water, and while the amount dissolved in each gallon 

 is very small, it extracts enough to provide for the 

 slow growth of the shell. ■ It is very important that 

 the shell be built up as rapidly as possible, for the 

 oyster has many enemies continually on the watch for 

 thin-shelled specimens. In the lower part of the bay 



