Geographical Distribution of Mollusca. 



357 



On the present moss occurs — Succinea elegans var. ochracea 

 Betta., associated sparingly with — 



Vitvea crystallina MiiU. | Limnaea truncatula Miill. 



The surface of Hale Moss, as that part of the old lake is 

 called to-day, which is not under cultivation, is well sprinkled 

 with tufts of the rare plant Schoenus nigricans L. Between 

 the tufts the marl lies bare without any peat. In wet weather 

 the Succinea crawl freely on the surface, and in drier weather 

 burrow beneath the marl, or crawl into the roots of the Schoenus. 

 In winter they are to be found in hibernation, securely fixed to 

 the ' rushes ' some inches above the base of the stem. This 

 Succinea is unlike any other British form. The peculiar extended 

 spire, and the angularity of the last whorl would seem to place 

 it as intermediate between the two species 5. elegans and 

 S. oblonga. It may perhaps be due to xerophilic conditions, 

 marking an advance from an amphibious to a land mollusc. 



§ 2. Calcareous Rivers and Streams. 

 No good example in the district. 



§ 3. Non-Calcareous Lakes and Ponds (Soft-water). 

 I. Ponds. 



Open Water. 



LiiiDicpa pevegev ^liilL 



Reed Belt. 



Marsh. 



S jyhcerimn corneum L. Limncea truncatula Miill. 

 Planorbis spirorhis Miill. 

 \Aplecta hypnorum L. 

 I Pisidiuni pusilliim 

 1 Gmelin. 



\Siiccinea elegans Risso. 



This association is typical of a non-calcareous pond with 

 very little reed belt, and a marshy tract at one end which 

 becomes in summer a dense mass of high grass and sedges. 



2. Very shallow ponds :- 



Open Water. 

 SphcBviimi lacitstre Miill. 



Reed Beet. 

 Nil. 



Marsh. 

 Nil. 



Such ponds often become practically dry in summer, only 

 moist clay remaining. Sph. lacusire, a species with very closely 

 fitting valves, is w^ell adapted for retaining life even under 

 these conditions, and may be found buried in the dry mud. 



1909 Oct. I. 



