210 LAKES AND RIVERS. 



smaller than the female. The shell is oval, not so 

 glossy as P. contecta. The variety unicolor is without 

 bands. This species and the preceding inhabit slow 

 rivers and canals. 



Bythinia (Gray) has the eyes sessile, the oper- 

 culum testaceous and solid, and the nucleus nearly 

 in the middle. The chief difference between this 

 and Paludina is that Bythinia is oviparous. The 

 eyes are sessile instead of being placed on stalks. 

 Although the name would indicate that they inhabit 

 deep water, yet they generally frequent shallow 

 streams, canals, or ditches. B. tentaculata has the 

 body nearly black on the upper part, while below it 

 is yellowish ; the head is small, the eyes large. The 

 form of the shell is subconical, or oval, of an amber 

 or brownish colour, opaque, and glossy; the length 

 is 0-5 ; the breadth, 0-25. The varieties are ventri- 

 cosa, with a white shell ; decollata, with the upper 

 whorls wanting ; excavata, with the whorls rounded, 

 and suture deeper. It is said to feed partly on animal 

 substances. B. Leachit (Sheppard) has a conical shell, 

 thin, and semi-transparent, horn, or amber colour, very 

 thin epidermis ; the operculum is flat, and nearly 

 round ; the length is 0*25 ; the breadth, 0*2 ; the 

 body is grayish-white, with yellow specks. 



Hydrobia (Hartmann) has the eyes on tubercles, 

 and a thin, horny operculum. These small molluscs 

 live chiefly in fresh water, but H. ventrosa is found in 

 estuaries, and pools near the shore. In Britain 

 there are only two species of Hydrobia, similis 

 and ventrosa. In both the shell is semi-transparent, 

 usually horn colour, but in similis sometimes white, 



