WATER SNAILS. 
219 
nitidus is depressed, the upper more convex than 
the lower side; of a dark horn colour, very 
glossy, semi-transparent; the whorls are four in 
number; the diameter of the shell is two and 
a half lines; the outer whorl exceeding the rest 
in size, with a blunt keel in the middle. 
This small and pretty mollusk is not frequent, 
though dispersed throughout the British isles; 
it may be obtained generally on the fallen leaves 
of trees, also on aquatic plants in ponds and 
ditches. 
Planorbxs lineatus —(;the Streaked Coil Shell) 
(PI. XI., fig. 182).—The shell of this species 
closely resembles that of the last, but is of a 
lighter colour, thinner, flatter, the keel sharper, 
and the last whorl not so embracing; it is 
markedly distinguished by the presence of from 
two to five curved transverse plates inside the last 
whorl; on the exterior these septa appear as 
whitish lines. This peculiarity of structure in¬ 
duced Liglitfoot, in 1786, to call this shell a 
Nautilus; and subsequently Dr.Fleming regarded 
the presence of the plates as sufficient to raise it 
to the rank of a genus, under the name Seg- 
mentina . 
The present species is the rarest of the British 
fresh-water mollusks; it is found chiefly in the 
neighbourhood of London and in the adjacent 
districts; it occurs in the co. Tipperary, Ireland; 
and is distributed throughout Central Europe. 
