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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 II. PULMONOBRANCHIATA. 



Family. LIMN^ID^. 

 Genus^ I. PLANORBIS. 



1. P. LINEATUS (streaked). 



Quoit-sha.ped, compressed, more convex above than 

 below, thin, semi-transparent, brownish or reddish horn- 

 colour, carinated; whorls f bur ; spire sunk; umbilicus narrow, 

 bnt 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. NITIDUS (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 much as 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 



