6 
SYSTEMATIC DISTKIBUTION. 
In describing shells, they should be regarded In 
their natural position ; that Is to say. In the way In 
which they are placed on the animal. Thus the part 
of the shell over the head of the animal Is called the 
front, and that over the tall the back, of the shell ; 
and the left and right sides of the shell correspond 
with the left and right sides of the animal. 
This Is exceedingly easy to be determined in the 
univalve shells, because the apex of the shelly cone, 
whether it be simply conical or spiral, in all univalves 
(except Patella and Lottia) is over the hinder part of 
the animal; therefore, if a shell is placed on its mouth, 
with the apex towards the spectator, the parts of the 
shell will correspond with the position of the person 
who is looking at it. 
As all shells are formed of a shelly cone (which, 
when very long, is generally rolled round an imagi- 
nary axis, for the purpose of diminishing the space 
that it occupies ; but when it is short, is sometimes 
only slightly recurved, as in the Ancylus and VeU 
letia\ in order to maintain a similarity of terms for 
the same thing in these two forms, all the lines or 
grooves which pass from the apex of the cone to the 
mouth, and which are caused by some permanent 
modifications of the edge of the mantle, are called 
longitudinal or spiral, and all the lines which are 
paralled to the edge of the mouth of the shell, and 
which, in fact, are generally marks of its growth, or 
are caused by some periodical development of the 
margin of the mantle, are designated as concentric 
or transverse. Thus the stride on the Cyclostoma 
elegans and Planorhis albus are longitudinal or spiral. 
