28 LIMNAID A. 
It need never be mistaken for a Planorbis, the 
general form of which it resembles, as it is furnished 
with an operculum. Its mouth, too, is perfectly cir- 
cular, which is the case with no species of our English 
Planorbis. 
Order IT.—PULMONOBRANCHIATA. 
Family.—LiMN HIDE. 
Genus I.—PLANORBIS. 
1. P. LtinzAtus (streaked). 
Quoit-shaped, compressed, more convex above than 
below, thin, semi-transparent, brownish or reddish horn- 
colour, carinated; whorls four; spire sunk; umbilicus narrow, 
but deep. Its special characteristic is the nautilus-like 
septa which are visible from the outside of the shell at 
intervals across the whorls. 
This interesting species is local; it is found in 
sluggish streams and ponds in several parts of England, 
and also in Ireland. 
2. P. nitipus (shining). 
Quoit-shaped, much depressed, thin and glossy, yellow- 
ish or reddish horn-colour, sharply keeled; whorls four to 
five ; spire sunk, but not so muchas that of the last species ; 
umbilicus small and shallow. 
P. nitidus may be distinguished from P. lineatus in 
having no septa, being more depressed, the spire not 
: 
| 
| 
| 
