T50 



MADRAS FISHERIES BULLETIN 



\'OL. XIV. 



species both are conspicuously toothed. Some have a superficial 

 likeness to the little Nassid Arcularia ; the form of the mouth is 

 however quite different. 



Of fresh-water snails belonging to this division, Limnaea and 

 Planorbis are typical, the one elegantly spiral, and the other dis- 

 coidal, coiled in a flat sinistral manner. The shell of Limnaea is 

 thin and fragile, with a large body-whorl ; the colour brown. In 





I'IG. 36. Pldnoibis crawling on water weeds {Vallismria). X I. 



Planorbis, usually a much smaller snail, and nearly black, the 

 shell is stouter. It is specially remarkable because a lobe of the 

 mantle just outside the pulmonary cavity has been metamor- 

 phosed into a functional gill ; this genus therefore, derived from an 

 air-breathing land pulmonate, which in turn originated in a water- 

 breathing gilled marine gastropod, has returned to a wholly aquatic 

 life, and has re-adapted its breathing apparatus a second time. It 



