110 GASTEROPODA. LIMNADZ. 
Shell diaphanous, horny, glossy ; the whorls very finely stri- 
ated transversely. Animal testaceous, freckled, and varied with 
olive-green. 'Tentacula and the margins of the foot testaceous, 
and most frequently without dots. Foot elongate and acumi- 
nate. Eyes very black. The reflected portion of the mantle 
often tinted with olivaceous. Height half an inch: breadth of 
aperture three-eighths. 
P. fontinalis is extremely common, and is generally diffused 
over every part of Britain. It occurs in fountains, and slowly 
flowing waters; and is very rarely found in stagnant pools. 
The movement of the animal in the water is slow. When at 
rest, it frequently settles on aquatic plants, near enough to the 
surface to be enabled to respire at will; and when it is an- 
noyed by the approach of wandering animals, it repulses them, 
with repeated blows, inflicted by a rapid movement of the shell ; 
the foot being the point of fixture. 
The last whorl of the shell, when the animal is in it, is ele- 
gantly reticulated with blackish olive, caused by the colouring 
of the mantle, which is visible through its pellucid shell. 
Genus 49. NAauTaA. 
Testa alt? acuminato-turrita; spira ad apicem obtusiuscula. 
Umbilicus nullus. Pallium non dilatatum. Tentacula seta: 
cea, ad basin interné dilatata, oculigera. 
Shell highly acuminately-turrited; the apex of the spire 
rather obtuse. No umbilicus. 
Mantle not dilated. Tentacula setaceous. 
1. Nauta Hypnorvum. 
N. testa ovato-oblonga, diaphana, cornea, polita, nitida ; an- 
fractibus tenuissimé transversim striolatis. 
Bulimus Hypnorum, Brug. Encycl. Méth. Vers, 301, 11. 
Planorbis turritus, Mii/l. Verm. Hist. 354. 
Bulla Hypnorum, Mont. Test. Brit. 228; M. & R. Trans. 
Linn. Soc. viii. 127; Dill. Desc. Cat. 488; Turt. Conch. 
Dict. 26. 
