STEI.FOK : PISIOItlM FATTNA OF THE r.RAND JUNCTION CANAL. 29I 



I suggested to Mr. Oldham that gatherings should l)e made from 

 certain well-marked and circumscribed habitats, so that we might be 

 in a position to compare from ciifferent stations the various forms that 

 live in association with one another. The bed of the canal is for the 

 most part stony, but here and there, particularly at places just above 

 lock-gates, there are deposits of chalky mud, sometimes several feet 

 in depth. The surface layer of these muddy patches is in a constant 

 state of disturbance owing to the frequent passage of boats — many of 

 them self propelled — and the currents set up by the opening and 

 shutting of the lock-gates. This agitation probably accounts for the 

 paucity of plant life on the muddy patches; but a few plants do 

 occur, the most common being Potamogeton pectinatus^ P. perfoliatus, 

 and Elodea canade?isis. These patches of mud are the chief strong- 

 holds of the Pelecypod fauna of the canal, and obviously offer more 

 convenient collecting-grounds than the stony bed in other parts. 

 Besides the Pisidia occur Sphcerinm rivicola^ S. corneuin, S. lacttstre, 

 Afwdonta cyg?tea, Unto pidontm, and U. iumidus. The five stations 

 from which the material was obtained were areas of a few square 

 feet in patches of mud at the following places : — 



(i). Above lock, S.W. of Berkhamsted railway station (Herts.). 



(2). Above the Cowroast lock, Dudswell (Herts.). 



(3). Above lock, N. of Wilstone village (Herts., Aylesbury branch 

 of canal). 



(4). x'lbove lock, S. -of Hospital Farm, Marsworth (Bucks.). 



(5). N.W. of Marsworth Church (Bucks.). 



The swirl caused by the working of the locks has its disadvantages, 

 for it brings together various varieties of several species {e.g., of 

 P. casertajium and P. nitiduj/i), which probably would not, in a state 

 of nature, be found in association. In rivers like the Thames this 

 state of affairs is naturally to be met with, but the occurrence of it in 

 the canal made us think at one time that the extreme forms of P. 

 caserta7iuin and P. nitiduvi nmst be distinct species, owing to the fact 

 that they were found to hve with practically typical examples of their 

 respective species. Further research has made us feel justified in 

 believing that they are varieties produced by their having lived under 

 various conditions of environment and that they have come to asso- 

 ciate with their respective types by some disturbance of the canal 

 bottom, possibly by the action of the self-propelled barges which are 

 now in common use on the canal. 



The origin of these well-marked varieties which are found in asso- 

 ciation is obscure, but a possible explanation is that at some places 

 in the canal decaying vegetable matter collects, whilst at others 

 occur shifting deposits of chalky mud or silt with scarcely any vcge- 



