Teeth of pond snail. Limnasa caperata. c, central 

 tooth; 1-7, lateral teeth; 8, 9, intermediate teeth be- 

 tween lateral and marginal teeth; 10-25 marginal 

 teeth. Greatly magnified. 



for a time, and then make some new ones. Compare these with 

 the first ones and you will doubtless find a vast difference. After 

 repeated examinations you will ultimately secure a perfect drawing 

 and description." 



After examining several more radula) the Professor placed several 

 types of jaws under the microscope for our observation. These were 

 horny organs of various 

 shapes, frequently armed with 

 ribs or other projections. 



When asked how many 

 teeth a snail had on its rad- 

 ula, Professor Parker direct- 

 ed us to figure the number 

 for ourselves from the radula 

 of the white-lipped snail. This 

 we did in the following man- 

 ner : We first counted the 

 teeth in one transverse row, 

 beginning with the central 

 tooth and counting each way toward the margins, and found eighty-nine 

 teeth. Then counting the rows of teeth vertically we found that there 

 were one hundred and twenty rows. Multiplying eighty-nine by one 

 hundred and twenty we secured the enormous sum of ten thousand 

 six hundred and eighty, the number of teeth in the mouth of a single 

 snail ! 



Professor Parker told us that the number of teeth varies greatly 

 in the different classes of snails. For example, a sea slug, ^Eolis drum- 

 mondi., has but sixteen ; the common 

 whelk, Buccinum undatum, has two 

 hundred and forty; and another sea 

 slug, Doris tuberculata, has six thou- 

 sand; the edible snail of Europe, 

 Helix pomatia, has twenty-one thou- 

 sand ; while another snail, Helix gkies- 

 Itreghti, has the enormous number 

 of thirty-nine thousand five hundred 



J 



and ninety-six ! 



George asked the Professor how 

 it was possible to study the radula 



of some of the minute snails, like the Pupa shells. He replied that 

 these must be boiled in caustic potash, and that he would prepare 



43 



Jaws of land snails. Upper figure, Poly- 

 gyra thyroides, a common land snail; lower 

 figure, Limax flavus, a common European 

 naked mollusk, or slug. Magnified. 



