230 
LIMNiEADiE. 
This species was first recorded as British by the 
industrious Petiver {Gaz. t. 10. £ 8.), who found it 
at Mitcham^ Surrey. 
Tentacles elongate^ linear ; jaws three, la- 
teral, rudimentary (fig. 57.) ; shell discoidal, 
(Planorbina.) 
8. Planorbis. (Coil Shell.) 
Animal (fig. 56.) 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 lu- 
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 
Fig, 56 . 
Planorhis albus. 
