90 



THE LOCAL EXTINXTION OF MOLLUSCS. 



SPECIES. 



Helix lamellata .. 

 Pupa vingens 

 Helix cavthusiana 

 Clausilia rolphii . . . 

 Vertigo suhsU'iata 

 Zonites fulvus 

 Cyclostoma clegans 



Helix avhustovuni 



HAST DISTRIBUTION 



Frequent 



do. 

 Occurs 



do. 

 Frequent 

 Occurs 

 Common 



do. 



H. acnleata 

 Acme lincata 

 Vertigo antivertigo 

 Cochlicopa tridens 

 Zonites ptinis 



PRESENT DISTRIBUTION. 



Locally extinct. 

 do. 

 do. 

 do. 

 do. 

 do. 

 ... Only a small colony 



existing. 

 Very rare ; probably 

 becoming locally extinct, 

 do. do. 

 do. do. 

 do. do. 

 Rare 

 Rare 



Occurs 



Frequent 



Occurs 



Frequent 

 . . Common 

 Turning our attention to the Freshwater Mulluscs, other 

 agencies affecting extinction may be assumed to come into play. 

 Reduction of quantity of suitable food may be one factor, but of 

 this I have no good evidence to offer. As regards lowering of 

 temperature, something may be said. One species, at least, 

 found readily in the ancient river alluvium, is now rare or locally 

 extinct, although it still flourishes as the water becomes deeper 

 towards the estuary. This is Neritiiia fliiviatilis. The conditions 

 under which the alluvium was laid down were those of stiller 

 and deeper, and therefore, perhaps, warmer water ; i. e., not so 

 leadily affected by cold springs, which now appear to act as a 

 deterrent of both animal and vegetable life, and of which I 

 could instance many examples.' Other species have been known 

 for many years in the lower reaches of the river, but they have 

 been unable to make any progress up stream. Two or three 

 species deserve particular attention among those that are dying 

 out. These delight in ascending ditches that have an immediate 

 CDmmunication with the river. Such are Succinea putris and 

 S. clegans, LimncBa palustris, L. ti'iincatiila, Planorbis carinHtus, 

 P. contorta, &c. The Planorbis will flourish, even though their 

 river-communication be cut off, but my experience is that 



6 In the shell-marls and peat which, as shown in the paper on the Felstead Shell-Marl 

 above referred to, were accumulating down to Roman times. They in this district all overlie 

 gravels. — J. F. 



7 [We have no evidence of any reduction of temperature in recent times ; but drainage and 

 the development of agriculture would certainlj affect plant life and the increase or decrease of 

 particular species. — T. V. H. As to evidence of reduction of temperature, Mr. Christy's 

 remarks, in his recent paper on " Essex as a Wine-producing County " (ante pp. 44-46) may be 

 usefully boine in mind. — Ed.] 



