SYSTEMATIC DISTRIBUTION. 
7 
and the lamelM on Helix lamellata and H. aculeata 
are concentric or transverse. 
But when we speak of the spiral shell as a whole, 
it is usual to call it short, or elongate, according to 
the length of the imaginary axis on which the 
whorls are rolled ; and when we speak of the 
length of the mouth, it extends from the line which 
forms the front, to the hinder edge of the mouth, 
which, in the Ancylus, occupies the whole length of 
the shell : the breadth is the line which crosses this 
at a right angle. 
From the manner of formation, most spiral shells 
are liable to four kinds of distortion : — 
1. The vertical elevation of the spire is more 
than normally depressed ; the shells are called 
subdiscoidal or depressed. 
2. The vertical elevation of the spire is more than 
usually elevated. When the whorls of these 
are continuous, these are called trochoid or 
subscalariformj and when they are separated 
from each other scalariform ; and when quite 
unrolled like a corkscrew, it is called ceratoid 
or cornucopia- shaped. 
3. The whorls are irregularly twisted ; this is most 
common in the genus Planorbis. 
4. When the whorls are rolled in a contrary di- 
rection to that of the usual or normal direc- 
tion of the shell, they are said to be reversed ; 
when the mouth is on the left hand of the 
axis, they are sinistral ; to the right, dextral. 
