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. 
PLANORBIS LINEATUS—(the Streaked Coil Shell) 
(Pl. 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 Lightfoot, m 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. 
