xxii INTRO D UCTION. 



manner, the lingual apparatus of our land and fresh- 

 water Gasteropods, and have therefore, in a few in- 

 stances, given the form of the jaw and number of 

 teeth as described in Tate's * British Mollusks.' 



The jaw and lingual ribbon of the larger species 

 may be easily extracted with a pair of finely pointed 

 scissors, the animal having been previously killed in 

 boiling water ; they should then be immersed in a 

 solution of caustic potash until all fleshy matter has 

 been removed, and afterwards thoroughly washed in 

 several changes of water. In the case of the smaller 

 species it is necessary, after killing the animal, to 

 remove the head and place it in a test tube with 

 caustic potash. After a few days the solution will 

 have become turbid, and should be carefully poured 

 off ; this is best effected by filtering it through blotting- 

 paper, which, when all the liquid has run through, 

 must be examined to make sure that the jaw and 

 lingual ribbon have not escaped from the tube, into 

 which a fresh supply of potash solution must be 

 poured ; this process is to be repeated from time to 

 time until all fleshy matter is removed. A much 

 quicker and less tedious mode of procedure is to 

 boil the solution by placing the test tube over a 

 spirit lamp, but in this case great caution is neces- 

 sary, as the potash is very apt to boil over and 

 destroy almost everything with which it may come 

 in contact. 



Shells. — It has already been stated that the shells 

 of molluscous animals are formed by a secretion of the 

 mantle, and that their composition consists chiefly of 

 carbonate of lime, with a small proportion of animal 



