2 4 THE OYSTER. 



shell might be obtained in the same way, and that the 

 whole history of the process might be traced by study- 

 ing them. At my suggestion, one of my students 

 put into the shells of a number of oysters thin glass 

 circles, such as are used to cover microscopic speci- 

 mens. The oysters were then returned to the water, 

 and were left undisturbed until new shell began to be 

 formed on the glasses. These were then taken out and 

 studied under the microscope. 



At the end of twenty- four hours the glass was found 

 to be covered by a transparent, faintly brown film of 

 thin gummy deposit, which exhibited no evidences of 

 structure, and contained no visible particles of lime, 

 although it effervesced when treated with acids, thus 

 showing that it contained particles too small to be 

 visible with a microscope. The gummy film is poured 

 out from the wall of the mantle, and in forty-eight 

 hours it forms a tough leathery membrane fastening 

 the glass cover over 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 ^V<j f an mcn l n g- 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 



