6 LAND AND FRESH-WATER MOLLUSKS. 
common term “snail”—in Latin, helix. Now 
the name employed to designate this assemblage 
of species is the generic one. We distinguish 
“the garden snail” by a further name, which is 
the specific one, because it points out the species 
or particular snail among the general assemblage 
of snails. Thus the garden snail is known 
among naturalists as Helix hortensis; that is, 
Helix, a snail, and hortensis, of the garden. 
The second species is Helix hispida, the hairy 
or hispid snail. 
A difficulty that meets us early in our exami- 
nation of these animals, is the variation of form, 
colour, and size among them. Now in the case 
of the oyster, we are all familiar with the nu- 
merous variety of shapes that the shell of this 
mollusk presents, yet we do not fail to recognize 
them as belonging to the mollusk in question. 
In the case of less familiar species, the difficulty 
increases ; and we must here try to form a series, 
with the normal or usual form as a centre, so as 
to include the extremes of variation. In most 
cases this is practicable. In a few instances 
varieties have been produced, which have be- 
come fixed or permanent; that is to say, the pe- 
culiarities which distinguish the variety have 
been perpetuated or handed down from genera- 
tion to generation. 
The principal causes of variation in shells are 
