290 JOURNAL OF CONrHOLOGY, VOI,. 1 5, NO. ID, SEPTEMBER, I918. 



opinion, referable to a robust form of that species, though sent to me 

 as the young of P. supititim. 



In ]w\y Mr. Oldham sent me examples of P. supinuvi from the 

 Grand Junction Canal at Dudswell, near Tring, Herts., amongst 

 which I detected a single example of P. parvubim. During the next 

 few weeks tie sent me many shells from Dudswell and Wilstone in 

 Herts., and from Marsworth and Cheddington in Bucks. At each of 

 these places P. parvnhcm occurred in association with P. supimnn 

 and other species of the genus, and we were convinced that Mr. 

 B. B. Woodward had been mistaken in referring our Irish shells to 

 the young of P. supinum as he had done (in litt.). At a later date, 

 and after further examination, Mr. Woodward agreed that some of 

 our Irish specimens were referable to P. parvulum^ but maintained 

 that the shells from the Grand Junction Canal and from the Pleisto- 

 cene deposits of the Thames Basin were clearly young P. supinum 

 (see Ann. Mag. Nat. Hist., ser. 8, vol. xviii., pp. 346-348, 1916). To 

 my mind P. parvuhwi even in its most robust form does not closely 

 resemble the young of P. S7ipinum, and it is strange that the two 

 should ever have been confused (cf. fags. 14-21, with 22-25, P^- 8) j 

 but proof of their diversity, if it were required, exists in the fry which 

 we have taken from the shells of P. parvulum, collected at different 

 places in the canal, the parent shells being no larger than very 

 juvenile P. supinum. 



Among the shells sent to us by Mr. Oldham were several which at 

 the time we could not identify, although we inclined to the belief 

 that they were merely thickened forms of common species, analogous 

 to the thickened forms reported from the Pleistocene of the London 

 Basin, and which still live in the Thames. So peculiar a facies had 

 the Pisidia from the canal, that it seemed worth while to investigate 

 them on ecological lines. Mr. Oldham undertook to obtain the 

 necessary material, and during 19 17 collected extensively in several 

 carefully selected and restricted habitats in the canal. He forwarded 

 many of his takings to me alive, and whilst the task of working out 

 the identity and affinities of the different species has fallen to me, I am 

 permitted to say that the views expressed in this paper are shared by 

 Mr. Oldham and Mr. Phillips, with whom I have been in constant 

 touch during its preparation. 



The Grand Junction Canal was constructed about the year 1800. 

 Leaving the Thames at Brentford, it runs northward up the valley of 

 the Colne, and after crossing the Chilterns, traverses the drainage 

 areas of the Great Ouse and Nene. The main sources of its water 

 supply are the large reservoirs near Tring, and streams in the systems 

 of the Colne, Great Ouse, and Nene. 



