256 PNEUMONOBRANCHIATA. 



Shell half an inch long, and a third part as broad, 

 dark horn-colour, glossy and transparent; spire com- 

 posed of five or six produced and hardly raised volu- 

 tions, ending rather acutely; aperture narrow-oval, 

 covering about half the shell ; the pillar a little sinu- 

 ate, often of a pale rose-colour. 



The eggs are deposited in oblong masses, exactly 

 similar to those of Limnceus, slightly attached to shells 

 and water-plants. (See Pfeiffer, t. 7. f. 24. 27.) 



This species was first recorded as British by the 

 industrious Petiver (Gaz. t. 10. f. 8.), who found it at 

 Mitcham, Surrey. 



* * * Tentacles elongate, linear ; shell discoidal, sinis- 

 tral. (Planorbina.) 



6. Planorbis. (Coil Shell.) 

 The animal with a small foot; tentacles with an 

 auricle at the base, and a long slender subcentral 

 spiral body, which is covered with an external 

 discoidal dextral shell, the whorls rolling nearly 

 on the same plane ; with a lunate or subquadrate 

 mouth, and a simple cavity. 



In considering these shells as sinistral, the spire is 

 that side which is uppermost when the mouth is 

 placed on the right side of the spectator, with the 

 most expanded part of the outer lip downwards. 

 It is important to observe this distinction, for Dr. 

 Turton and others, in describing the species, have 

 sometimes called the under side the upper, and vice 

 versa. 



This genus is so named from the flattened and ho- 

 rizontal coil of the volutions, by means of which the 

 whole of the gyrations may be seen on each of the sides. 



