52 HELICIDA. 
Genus ITI.—ZonitTEs. 
The identification of the different species in this 
genus gives more trouble to the novice—to whom they 
all look pretty much alike—than all the rest of British 
land and freshwater shells put together, with the ex- 
ception of the minute Vertigos. When the adults of 
two species differ considerably in size, as radiatulus 
and nitidus, and the markings and sculpture are much 
alike, it is a good plan to count the whorls of a small 
specimen. A full-sized radiatulus has four and a half, 
and nitidus five. If, therefore, the shell under con- 
sideration is the size of radiatulus, and has three whorls, 
the observer will know that another whorl and a half 
would make the shell too large for a radiatulus, and he 
may conclude it to be a young nitidus. 
The animals should be carefully compared. Of the 
ten species, fulvus, crystailinus, purus, radiatulus, and 
excavatus are unmistakable when their striking diffe- 
rences have been once pointed out. 
1. Z. cennArius (inhabiting cellars). 
Compressed, glossy, semi-transparent, pale horn-colour 
above, opaque white below; spire nearly flat; mouth deeply 
semi-lunar; whorls five to six; umbilicus broad and deep; 
diameter half an inch. 
