Geographical Distribution of Mollusca. 



355 



As is usual, apparently, on the more open grounds, the typical 

 form of L. arboriim is replaced by the darker deeply-banded 

 variety. It is fairly common in the vicinity of moisture, and 

 exhibits the gregarious habit, five or six examples often con- 

 gregating under one stone. The above species were taken with 

 it at an altitude of about looo feet, but occurred only spar- 

 ingly. V. aUiaria, a species which seems able to adapt itself 

 to almost any conditions, finds refuge in dry weather under 

 stones in boggy places. 



MARSH LANDS OR WET GRASSLANDS. 

 S I. Calcareous Lakes and Ponds (Hard- water). 



Open Water. 



Limncsa pereger Miill. 

 Planorbis albus L. 

 Valvata piscinalis IMiill. 



Reed Belt. 



Physa fontinalis L. 

 LimncBci palustris Miill. 

 Planorbis contortus L. 



,, /oM/a;; i/sLightfoot. 

 Bithynia tentacitlata I.. 

 Valvata cristata Miill. 

 Pisidiinn uitiditm Jen. 

 mil i urn Held. 



Marsh. 



L. triDicatula Miill. 

 Aplecta hypiioniii! L. 

 Pisid. piisilliiii! Gmcl. 



,, obtusale Pf. 

 Carychiitm minimum M. 

 Vertigo pygmcea Drap. 

 Pinictiim pvomcpum 



Drap. 

 C. liibrica IMiill. 

 Vitrea radiatiilata Aid. 

 crvstalliiia Miill. 

 Euc. fidvus Miill. 



Schoenus Belt. {Schaomis 



ocJiracea Betta. Associated with 



Between Reed Belt and Marsh. 

 nigricans L.). 



Siiccinea elegans, \'ai 

 Limncea iruncatula Miill. 



The above are the molluscan fauna associations of a typical 

 calcareous lake. Neritina flitviatilis and Ancylns ftiiviatilis 

 are absent, being typical denizens of the non-calcareous rivers. 



The former existence of large expanses of water in the dis- 

 trict is shewn by the extensive development of lacustrine marls 

 at Burton and Silverdale. The contents of the marls strongly 

 confirm the present associations of the calcareous lakes, and 

 throw some interesting light on former conditions of aquatic 

 life. 



Havves Water at Silverdale, with its beach of exposed 

 chara-marl, is still the largest sheet of water in the calcareous 

 region. The species of the open water are not numerous. 

 The present LimncBa pereger are of the usual lacustrine type, 

 but those in the marl fall into two groups, one lacustrine and 

 the other a form similar to that common to calcareous streams 

 at the present time. Physa fontinalis is absent in the marl, 



igoij Oct. I. 



