66 LAND AND FRESH-WATER SHELLS. 



mouth circular, with a complete peristome ; umbilicus deep ; 

 operculum circular, greyish-white. Length | inch. Lakes, 

 ditches, canals, and rivers. V. piscmaKs. 18 



(b) Shell flatly coiled pale horn coloured, glossy ; whorls 

 five, the last one very large ; mouth circular with a continuous 

 margin ; umbilicus large and open, exposing the interior con- 

 volutions ; operculum round, reddish-horn coloured. Diameter 

 iVh inch. Lakes, ponds, canals, and ditches. V. cristata. 



ORDER II. PULMONOBRANCHIATA. 



Shell generally spiral and external, but sometimes (Limacidce) 

 rudimentary and internal, * or wanting. Body spiral, generally 

 non-operculated, but sometimes with an operculum ; respiration 

 effected by means of a lung. 



Family I. Limnseidse. 



Shell spiral or hood-shaped ; mouth without teeth. Tentacles 

 two ; eyes sessile. 



6. Planorbis. Shell orbicular, flat, coiled nearly in the same 

 plane; mouth semi-circular; umbilicus distinct ; non-operculated. 

 Tentacles two in number, very long ; eyes sessile ; foot oval, short. 



(a) Shell quoit-shaped, depressed, yellowish-horn coloured, 

 glossy ; whorls four with two to five curved transverse plates 

 inside the last whorl, which appear as whitish lines when viewed 

 from the exterior ; periphery bluntly carinated ; umbilicus narrow, 

 deep. Diameter |th inch. Ponds and slow streams. Local. 



P. lineatus. 19 



(b) Shell quoit-shaped, thin, glossy, yellowish-horn coloured, 

 more or less reddish ; whorls four to five, the outer whorl being 

 without septa as in P. lineatus, and very large in proportion to the 

 rest, covering one half of the preceding whorl, and bluntly keeled 

 in the middle of its periphery ; spire not so much sunk as in P. 

 lineatus ; umbilicus small, shallow. Diameter 2| lines. Ponds 

 and ditches on aquatic plants. P. nitidus. 19 



18 v. depressa, more depresssed, umbilicus large ; v. albina, white or 

 whitish; v. pusilla, smaller, striae, stronger whorls four and a half; v. 

 acuminata, spire produced, apex sharp. 



v. albina, milk-white, transparent. 



19 v. albida, white ; v. minor, smaller. 



