146 BRITISH MOLLUSKS. 



and they are more truly discoid, the upper and lower disk of the 

 shell being almost equally concave, and each showing impressed 

 sutures. The aperture varies in outline according to the greater or 

 less development of keel on the edge of either disk. 



9. Planorbis contortus. Twisted Planorbis. 



Shell ; depressed, brownish horny, upper disk flat with the sutures 

 impressly channelled, lower disk broadly exca- 

 vately umbilicated ; whorls six to seven, extremely CZD 



closely coiled one upon the other, finely striated, 

 rounded at the periphery, gently sloping to the 

 edge which is obtusely angled ; aperture small, 

 compressly lunar. 



Helix contorta, Linnaeus (1758), Si/st. Nat. 10th edit. p. 770. 



Planorbis contortus, Muller (1774), Verm. Hist, part ii. \. '^^^P' 

 p. 162. -"i*^ 



Helix crassa, Da Costa (1778), Brit. Conch, p. 66. pi. iv. 

 f. 11. 



Helix umbilicata, Pulteney (1799), Cat. Dorset. 



Planorbis (BathyomphalusJ contortus, Moquin-Tandon (1855), Hist. Moll. 

 vol. ii. p. 443. pi. xxxi. f. 24 to 31. 



Hab. Throughout Europe. Siberia. (In ponds and ditches.) 



The shell of this small Planorbis has the whorls more closely 

 coiled one upon the other than that of any other mollusk. Although 

 of truly discoid growth, the under surface is distinctly and largely 

 umbilicated ; the upper surface is very little concave, and the sutures 

 of the very narrow whorls are impressly channelled throughout. 

 The animal is dusky brown about the head and neck, black at the 

 foot ; and the tentacles are rather elongately setaceous, tapering to 

 a point. It lives among the roots and stems of water-plants, in 

 ponds and ditches throughout Britain, but it is local ; on the Con- 

 tinent it is also generally diffused, passing in an easterly direction 

 into Siberia and southwards to the islands of the Mediterranean. 



Planorbis contortus is sluggish and irritable, says M. Moquin- 

 Tandon, allowing itself to sink in the water on being touched. It 

 then rises with a slightly jerking movement, carrying its shell hori- 

 zontally, and on reaching the surface manages to let it float. 



