230 LIMNiEAD^. 



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; Jaivs three, la- 

 teral, rudimentary (fig. 57.) ; shell discoidal. 

 (Planorbina.) 



8. Planorbis. (Coil Shell.) 



Aninaal (fig. 6Q?) with a small foot; tentacles with 

 Fig. 56. 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 



rianorbis albus. ^^ ^^^ ^^^^^^ ^^^^^ . ^^-^jj ^ j^. 



nate or subquadrate mouth, and a simple cavity. 



The primary lateral tubercles of the teeth have 

 three apices; and the central tubercle generally in 

 the genus has two apices placed far apart from each 

 other (fig. 9. at p. 61.). This appears be merely 

 the result of the suppression of the third intermediate 

 apex, — a view in which I am borne out by a spe- 

 cimen of the tongue of P. marginatus, in which there 

 is only one side apex to the central tooth, the central 

 apex and that on the other side being both suppressed. 



In considering these shells as dextral, the spire is 

 that side which is undermost when the mouth is 

 placed on the right side of the spectator, with the 

 most expanded part of the outer lip upwards. It 

 is important to observe this distinction; for Dr. 

 Turton and others, in describing the species, have 



