8 JOURNAL OF THE [January, 



isfactory. Advantage was next taken of the resistance of the 

 radulae to the action of ordinary chemical reagents. The 

 resistance to acids and alkalies induced the early belief in the 

 silicious composition of the teeth, and it is only quite recently 

 that the fallacy has been eradicated from text books on natural 

 history, and from special works on the mollusca. It is now 

 known that the teeth are composed of a substance closely related 

 to chitine. Its behavior under the influence of the ordinary 

 staining fluids used in microscopical work, is quite varied and 

 interesting, and affords some points of comparison with true 

 chitine. 



Another method, applied in the study of the extremely small 

 radulae of minute species of snails, was to crush the animal, and 

 examine the dentition through the translucent tissues. Of 

 course, this plan is in itself not altogether satisfactory, on ac- 

 count of the difficulty of distinctly studying the characters of 

 the lingual membrane. Besides, it was not conducive to the 

 production of clean, beautiful and permanent preparations, such 

 as ought to be retained, to serve as the types from which descrip- 

 tions and illustrations have been made, and from which import- 

 ant deductions have been drawn. 



When the characters of the odontophore came to be studied, 

 it was first thought that they would furnish a simple means of 

 classification, and an infallible method of determination. At 

 the present time, the best authorities have abandoned nearly all 

 the classifications of the Gasteropoda based upon the characters 

 of this member alone, and give to it an importance about equal 

 in value to that of the shell. Thus it will be seen that the 

 radula still holds an important position in the study of the 

 mollusca, but is not of the greatest value. 



Several of the steps indicated in the following directions for 

 preparing the radulae of the Odontophora, for microscopical 

 study, and for permanent preservation, have been employed by 

 previous investigators in this department of research ; but it is 

 believed that some novel features are here described, and the 

 entire sequence of processes is reduced to a system, which will 

 be found to produce uniform and satisfactory results. At first, 

 I adopted the methods in common use, and found that for the 

 work which I had undertaken, namely, the study of the lingual 

 dentitions of the American fresh-water species of RissoidcB, I 



