90 THE RADULyE OF 



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

 still holds an important position in the study of the moUusca, 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 se- 

 quence 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 Rissoidce — I could not attain the 

 desired degree of excellence in the preparation of the radute, 

 which would enable me to make a complete study of their various 

 features. This led to a long series of experiments, performed with 

 all the principal reagents used in microscopical investigation. An 

 enumeration of these experiments would add but little to our 

 knowledge, beyond the fact that most reagents are useless for this 

 work, and many are of but little value. 



When manipulating with such small objects as the lingual rib- 

 bons of our Rissoidce, small species of Fianofbis, Goniobasis, Pi'J'o, 

 Vertigo, etc., simplicity is of the greatest moment, for in transfer- 

 ring the radula from one dish to another, and passing it through 

 successive reagents, it is very likely to be lost, or so mutilated in 

 handling as to be worthless. Therefore, a complicated or laborious 

 method is to be avoided if possible. 



The transparency of the objects is also another obstacle to be 

 overcome, and while mounting media can be selected of a 

 different refractive index, yet the loss of absolute and reliable 

 differentiation, from the reflection of liglit from tlie polished 

 denticles, the interference of perspective in media of low refractive 

 indices, and tlie diffraction lines produced by the minute denticles, 

 render it extremely desirable to stain the specimens, and to mount 

 them in a highly refractive medium, or one that nearly agrees with 

 the refraction of the objects themselves. 



MiiTHOD OF Preparation. ^The shells having first been 

 boiled or placed in alcohol to kill the organisms, the animals are 



