152 LAND AND FRESH-WATER MOLLUSKS. 
2 * ■. The central tooth in each is small: that 
of Z. lubrica is simple, whereas that of Z. lubri- 
coidea has a minute denticle on each side. 
Genus Azeca. 
Azeca and Zua are separated from Bulimus by 
artificial characters; for there are no structural 
differences between the animals of these three 
genera. The shell of Azeca in form and colour 
is like that of Zua , and is equally polished; but 
the aperture is furnished with folds or teeth, of 
which there are usually three. There is only 
one British species— 
Azeca tridens — (the Glossy Trident Shell) 
(PI. IX., fig. 85).—This snail is rare and sparingly 
distributed throughout England, and is more 
frequently met with on the chalky and oolitic 
soils of Suffolk, Essex, Kent, Surrey, Glouces¬ 
tershire, &c.; inhabiting the moss and dead 
leaves in moist woods, congregated in little 
parties of six to nine. It occurs in the Newer 
Tertiary deposits of Copford and Clacton. 
Genus Pupa (Chrysalis Snails). 
The animal is furnished with a short foot, 
acute behind; the tentacles are short, especially 
the lower ones; in the Sub- genus vertigo the 
inferior tentacles are obsolete. 
