THE LOCAL EXTINCTION OF MOLLUSCS. 89 



dew at night. This allowed the Molluscs an excursion daily, 

 and, in the long run, further peregrinations and a wider field for 

 the distribution of species. In the arable and drained tracts of 

 the present day, drought is much more prevalent, and dewy 

 nights are possibly not so frequent ; as a consequence, the snail 

 is much limited in its range and opportunities. In short, where 

 once the whole summer was humid, there are now only the 

 heavy dews of the spring and autumn. We cannot tell whether 

 the food is correspondingly diminished. There is a little 

 evidence in that some species frequent gardens where young 

 juicy vegetation is more predominant, but it is equally true that 

 many species do not go into a garden at all. 



A drained tract acts directly in breaking up colonies and 

 isolating individuals. All travel instinctively away from the 

 drained tract, and consequently, many travel in opposite direc- 

 tions, never to unite again. This sub-division goes on more and 

 more as the colonies diminish, till eventually they die out, from 

 want of opportunities to pair. There are two cases which are 

 obviously due to the effect of drainage, in which two species of 

 Mollusca, formerly very abundant in Essex, are now fast hasten- 

 ing to local extinction. One is Helix avbustovum, and the other is 

 Zonites punis. Both require to live in very moist situations. I 

 have been fortunate enough to hunt up three small colonies of 

 H. arbiistorum, and in one case I had seen the colony die out to 

 its last member. Zonites purus is now only found as single speci- 

 mens in my neighbourhood, and these are few and far between. 



The shell-marls and peat-bogs of this district, although not 

 extensive, are very instructive with regard to the proportions in 

 which the \ arious species are represented. Some of these marls 

 enclose Roman and other remains, and we can therefore learn 

 approximately their age. I here copy a table which I made out 

 for this journal in 1889,5 in which I have made a slight alteration 

 in the cases of H. arbustoyiim and Cyclostoma elegans, required by 

 the subsequent observations alluded to above. I believe that in 

 other respects it is correct. It will be seen by this table that 

 local extinction may be a somewhat rapid process at times : — 



5 '•On the Mollusca of the Shell-Marl occurring at Felstead and in other parts of Essex." 

 —Essex Naturalist, vol. ill., p. 14. 



