76 Proceedings 
class, from the earliest times down to the present. I may add 
that the class to be studied must not be of too small an extent, 
nor confined to a few regions of the earth. And there is only 
one class in the animal kingdom which meets these require- 
ments; it is the class Mollusca, consisting of snails and snail-like 
animals; the characteristic organ, rarely absent, being the shell. 
There is another organ characteristic of the Mollusca, found in 
practically all the species except the headless class; though we 
have no fossils which show this organ (except possibly a few 
Cephalopoda), it is of singular importance in classification. This 
organ is a tongue provided with many rows of chitinous hooks 
or teeth, It has been known since the beginning of modern 
Conchology, but until lately has not received the attention it 
deserves, Of course it cannot in any case be seen properly with- 
out the microscope. Our conchologists of fifty years ago made 
many careful estimations of the numbers of hooks or teeth oc- 
curring on the radulae of our different native species; but it 
was soon found that these estimates were only approximately 
correct, the actual number varying within rather wide limits. 
_ It was also noticed that there was a very great similarity between 
the radulae of allied species; and this fact, instead of stimulating 
interest in the subject, as it ought to have done, led to the radula 
being disregarded, since the main object of collectors has been 
to establish differences rather than similarities. It is, however, a 
mistake to suppose that the radula does not exhibit differences 
in closely allied species. The differences may be small, but they 
are perceptible, as you will observe in the drawings I now submit 
to your notice, which represent the radulae of our British Helices, 
a group that has until lately been included under one genus. 
Certainly there is a very great similarity between these radulae; 
but when they are examined carefully with a good microscope, 
due care being taken to mount them to the best advantage, they 
present the differences which you see in the drawings. These 
being slight, and the habits of the animals being to some extent 
known, these radulae afford us an excellent opportunity for 
speculating about the means whereby their present variety of 
form was brought about. The later geological history of these 
snails is continually becoming better known, chiefly through the 
