lO MOLLUSC A. 



imbricated or lapped layers built by the mantle border and 

 mantle outside of the division line, a continuous thin sheet 

 of shell deposited by the muscles {ads) , and an inner re- 

 gion deposited by the mantle inside of these limits, and 

 composed of layers which cover up the track, and are built 

 one upon another so as to add considerably to the thick- 

 ness of the interior. 



On the surface of a section it will be seen that there are 

 limited deposits of lime of a columnar structure, as in three 

 such in Fig. 14, sc, which are softer, and contain less animal 

 matter than the adjacent parts, and more widely distributed 

 layers. These appear to be due to some cause which irri- 

 tates the mantle at that particular spot, and causes it to 

 deposit wdth greater rapidity than usual ; at least this seems 

 to have been the cause in several cases observed by me. 



An excellent method to pursue in this connection is 

 to induce the children to weigh, and then roast, one of 

 the valves, and observe, subsequently, that the layers 

 of shell retain their form, but peel off readily, and be- 

 come much lighter in weight as well as more friable. 



A little of this burnt shell treated with acid effervesces 

 more readily than when unburnt, and is one of the 

 sources from which lime is obtained for agricultural 

 purposes on the sea-coast, where shells are abundant. 



The weight lost remains to be accounted for ; and 

 this can be approximately done by suspending a shell 

 of the same weight as the first in about a pint of dilute 

 acetic acid. In the course of a few days the carbonate 

 of lime — the mineral matter which makes the shell so 

 hard and heavy — is dissolved, and there remains the 

 animal matter. 



This takes the form of layers of charcoal lying be- 

 tween the lighter-colored layers of lime in shells, which 



