INTRODUCTION. 3 
place, and should any stranger wish for further informa- 
tion than can be obtained through the glass cases, the 
curator will always be ready to give courteous attention to 
his enquiries. 
Though the entire list of British land and freshwater 
species is limited to 130, there are numerous varieties of 
most of these; so that while a fair collection is within the 
reach of most people—often within the range of a single 
county—a perfect collection takes a long time to accu- 
mulate. 
The pleasures of collecting anything are too patent to 
need to be dwelt on here, but the pleasures of collecting 
objects of natural history in any branch have additional 
charms—the charms of the country. 
It should not be supposed that the summer is the only 
time when it is possible to collect. Throughout the winter 
much may be done, except, of course, when a frost puts an 
end to turning over stones, and drives all water shells into 
the mud. 
Doubtless every describer of shells has been puzzled to 
find simple geometrical terms suitable to their forms, which 
are often complex. I have followed the authorities in 
using such terms as ovate, subcylindrical, &c., though they 
are vague, and do not express to what degree the object is 
oval or cylindrical. Dr. Jeffreys calls L. stagnalis “‘ elon- 
gated,” which it certainly is; but what shape is “elon- 
gated”? ? This difficulty, however, affects the describer 
