214, LAND AND FRESH-WATER MOLLUSKS. 
and with a large and deep umbilicus; the colour 
is bronzed-brown or horn ; albino varieties occur. 
This minute snail possesses as many as 6,000 
lingual teeth. 
This is also a local species, though very 
abundant, when met with, living on water-plants 
in lakes, ponds, and ditches. 
Pianorsis vorrex—(the Flat Coil Shell) (Pi. 
XI., fig. 127).—The shell is extremely depressed, 
thin, brown, pellucid, glossy ; concave above, and 
flattish beneath, with from six to eight whorls ; 
the outer whorl is rounded, but flattened under- 
neath, so as to form a sharp keel or edge on the 
lower margin ; the diameter of the shell is three- 
eighths of an inch. 
This species is very generally diffused. It 
lives in shallow and stagnant waters, on the 
stems and leaves of plants, or floating on the sur- 
face, and rarely residing at the bottom. Its modes 
of progression are various, and very characteristic 
of the Coil Snails. It slowly glides along the 
surface of a leaf by extending its foot, and pro- 
ducing in it undulatory movements, by means 
of which it is propelled forward. Now and then, 
the shell, which is usually carried inclined at an 
angle of from 20° to 30°, is suddenly jerked 
forward by a semi-rotatory movement. Often: 
the snail moves along with a continuous progress, 
without jerking the shell at intervals. Some- 
