66 LAND AND FRESH-WATER SHELLS. 



culum horny. Body spiral, with two tentacles, provided with a 

 long plume-like gill which is protruded when the animal is 

 crawling ; eyes situated on the inner side of the base of the 

 tentacles. 



5. Valvata. 



(a) Shell globular, thin, solid, light horn-coloured, whorls five 

 to six, rounded, body-whorl very large ; spire obtuse, compressed ; 

 mouth circular, with a complete peristome ; umbilicus deep ; 

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

 ditches, canals, and rivers. V. pisdnalis^ 



(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 

 T ^th inch. Lakes, ponds, canals, and ditches. V. cristata. 



ORDER II. PULMONOBRANCHIATA. 



Shell generally spiral and external, but sometimes (Limacida) 

 rudimentary and internal, or wanting. Body spiral, generally 

 non-operculated, but sometimes with an operculum ; respiration 

 effected by means of a lung. 



Family I. Limnaeidae. 



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

 two ; eyes sessile. 



6. Segmentina. Shell orbicular, flat, with its cavity divided 

 into several chambers, each communicating with one another. 



(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. 



S. nitida 



7. Planorbis. Shell orbicular, flat, with its cavity entire. 



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

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

 acnminata, spire produced, apex sharp, 

 v. albina, milk-white, transparent. 



