spire-shells arid Marsh-snails 229 



far inland on the dry chalk-downs as well as keeping 

 up its connections by living along the coast-lands. A 

 glance at the figure will show that so far as externals 

 go it makes out a good case for being classified not 

 very far from the Winkles. There is the long 

 proboscis, the same contractile tentacles with eyes at 

 their base, the longitudinal division of the foot, and 

 an operculum, only this has become shelly instead of 

 horny. If we inspected the radula we should find 

 the number and arrangement of the teeth to agree 

 with that of the Winkles. But there is one great 

 difference : instead of the gill-plumes of the Winkles, 

 aeration of the blood in Cyclostor)ia takes place in a 

 chamber formed under the mantle 

 behind the head, the blood circu- 

 lating about the living tissues of 

 this chamber and taking up oxygen 

 from the air. This arrangement 



IT,. 1^7, 1. T Round-mouthed Snail 



led toth^iiJycLostoina being grouped 

 — until lately — with the land and fresh-water snails 

 that have a closed branchial chamber or lunof ; but it 

 is now recognised that there is a great difference in 

 the two. It is an exceedingly interesting case, for it 

 gives a distinct clue to the probable origin of other 

 land-forms that have got much more modified. It 

 must not be assumed that Cydosfoma originated in 

 this country from a Litforina. The genus is a 

 tropical one, and the probability is that C. elegans 

 had a southern origin, for it is found in the Canaries, 

 in Portugal, France, Italy, and Central Germany. 

 It extends no farther north than Yorkshire and 

 Donegal. 



Its habits are rather singular. A vegetable feeder, 



