GASTEROPODA. LIMNADZ. 113 
Shell horny, rather thick ; the whorls transversely striated ; 
the strize antiquated. Diameter one inch and a quarter. 
Animal of an intense olive-brown ; tentacula dilated at their 
base externally, gradually paler in colour towards their extre- 
mities. 
Colour of the shell brown, fuscous, or reddish-horn ; often 
whitish beneath. Spire composed of six whorls; the two first 
very small; the last very large. Three spires only are visible 
from above. 
Planorbis corneus is the largest of the European species. It 
is common in the marshes round London and Oxford ; and has 
been found by Montagu, near Wareham, in Dorsetshire. It is 
abundant likewise in ditches, near Eton and Cambridge. 
The animal, when teased, emits from its mantle a blood-red 
secretion, similar to that of the Aplysia in appearance. 
2. PLANORBIS ALBUS. 
P. testa griseo-corned, aut albida tenui; anfractibus longitudi- 
naliter elevato-lineatis. 
Planorbis albus, Mill. Verm. Hist. 164°; Flem. Edinb. Encyel. 
vil. 69. 
Helix alba, Mont. Test. Brit. 459; M. §& R. Trans. Linn. Soe. 
192; Dill. Desc. Cat. 909. 
Planorbis spirorbis, Drap. Hist. des Moll. pl. 2. figs. 8-10. 
Animal pallidé cinereum. Tentacula seepé dilutiora. Oculi 
atri. 
Habitat in stagnis, frequens. 
Shell grey horn-coloured or whitish, thin; the whorls with 
longitudinally elevated lines. Diameter five-sixteenths of an 
inch. 
Animal pale ash-coloured. Tentacula often paler. Eyes 
black. 
Shell thin, somewhat pellucid. Epidermis pale-brown, often 
fuscous. Whorls four; all visible from above and below ; with 
elevated, distant, transverse strize crossing the longitudinal 
lines, and producing a reticulated appearance. 
This species is common if ponds and ditches. It occurs 
