8 LAND AND FRESH-WATER MOLLUSKS. 
_ 350,000 different animals. But if he says that 
the garden snail is a molluscous animal of the 
class Pulmonifera, of the family Helicide, of the 
genus Helix,—by the first he excludes all the 
vertebrated, annulated, coral-like, and jelly-like 
animals from his comparison; by the second, he 
excludes the bivalves, the water-breathing snails, 
and cuttle-fishes; by the third, he distinguishes 
the garden snail from the slugs, pond snails, and 
the like; and having arrived at the genus to 
which it belongs, a few distinguishing characters 
in addition will enable him to determine the 
species. F'urther,ifthe classification be based upon 
natural and not upon artificial characters, then 
it expresses the relationship of the species ; for 
from the foregoing scheme of classification, it will 
be observed that differences in structure in the 
animals become less and less as we ascend in 
the scale of subdivision. Thus, for example, 
animals belonging to the same family, but of 
different genera, differ less from each other than 
animals belonging to different families. 
The subject will be treated according to the 
following plan :— 
The characters of each class will be studied by 
a full examination of the internal structures, as 
well as of the external or shelly parts, of a typical 
species of each. Technical terms, which we shall 
be compelled to use in describing each species,, 
