SYSTEMATIC DISTRIBUTION. 7 



and the lamella 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 

 subscalariform, 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. 



B 4 



