NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 45 



17. Wensleydale — Whitfield Gill near Askrigg, a single specimen picked 



out of moss, W.D.R. ! 

 20. Lower Tees — Ponds near Great Ayton, J. W. Watson. 



Subgenus GYRORBIS Agassiz, 



Shell very flat, more or less carinate, not umbilicated, smooth, whorls 

 progressively increasing, the last not much larger than the penultimate ; 

 aperture sub-oval, more or less angulated (Moquin-Tandon, 'Hist. Nat. d. 

 Mollusq. terr. et fluv. de France,' ii, p. 428). 



Planorbis spirorbis Miill. 



Planorbis Dazuri Morch. 

 Planorbis rotundatus Poiret. 



Widely distributed. 



According to Kobelt and Jordan Planorbis Dazuri of 

 Morch is identical with this species. Its northern Umit in 

 Britain is given by Dr. Jeifreys as the Moray Firth district. Its 

 range has now been extended to Caithness, Mr. Peach having 

 found specimens at Reiss and near Thrumster. 



This species is known on the Continent as Planorbis 

 rotundatus Poiret, the name spirorbis being appHed to Jeffreys' 

 variety ecarinata, which is considered a distinct species. 



It is probably the most widely diffused species of Planorbis 

 in Yorkshire, being found not only in the ditches, ponds, &c. 

 on the plains, but extending its range up some of the western 

 dales. At Malham Moss, on a plateau 1300 feet above sea 

 level, it lives in company with Limnaa truncatula, Succifiea 

 elegans, etc. 



It is recorded from North Australia by Mr. E. A. Smith, 

 who found two series thus labelled in the Cumingian collection. 



We have noticed that this species often lives in association 

 with Limncea glabra and Physa hypnorum, the same conditions 

 appearing to be favourable to all three species. This fact has 

 been confirmed by other observers. 



