BOYCOTT : THE HABITATS OF FRESHWATER MOLI.USCA. 24I 



The particular topic to which I should like at the moment to direct 

 your attention is the oecology of freshwater mollusca. Every practical 

 naturalist knows that different kinds of animals are apt to live in 

 different kinds of places, but it is only in comparatively recent times 

 that there has been any serious attempt to give precise answers to the 

 questions — (T)what is the characteristic habitat of each species?, and 

 (2) why is it characteristic ? The answer to the first question should 

 be comparatively easy ; it is a matter of observation, though obser- 

 vation as always must be tinged with provisional explanatory specula- 

 tion, which by trial and error, new observation and experiment may 

 be expected to bring us ultimately to the generalisations which will 

 constitute the second answer. Reference to our literature shows that 

 both questions have hitherto been neglected. 



The parish of Aldenham extends over some 6,000 acres in the 

 south of Hertfordshire, and I purpose to give a short account of the 

 occurrence of water snails in this area as nearly as pretty assiduous 

 attention during 191 5 and 19 16 has informed me of the facts. The 

 watery habitats presented by this district — and for its parochial 

 limitation there is no particular excuse beyond the necessity of some 

 sort of boundary — fall into four main groups : — 



{a), the river Colne in the north-west, with its accessory ditches 



and backwaters : 

 (/'), two lakes in the south, in intimate relation with one another, 



and ultimately with the river, through 



{c), a stream with four subsidiary branches ; a second small stream 

 is on the western boundary : 



{d\ iGt, f>ofids, mostly in the southern two-thirds of the parish on 

 the grass land of the London clay ; the north-western third lies 

 on chalk overlaid with gravel and clay, and is mainly arable and 

 pondless. Of these ponds we may provisionally distinguish 

 two groups : — 



(I.), rimning ponds through which there is always or at some time 

 of the year an obvious flow of water ; the former approximate to 

 streams, the latter to the next group ; of this sort we have 22 ; 



(II.), closed pojids in which there is at no time any patent stream 

 flowing in or out ; of this sort we have 141, of which 17 are 

 liable to be dry for a substantial part of an ordinary summer 

 ("drying ponds"), and 11 are too much humanised by ducks 

 to be usefully considered ; there remain 113 for examination. 



Since any final cecological classification of watery habitats will have 

 to take into account the whole of their biological, chemical and 

 physical characteristics, we need not as a preliminary spend much 



p 



