292 JOURNAL OK CONCHOI.OGY, VOL. 1 5, NO. lO, SEPTEMBER, I918. 



tation, and in consequence the environment or shells Hving within a 

 few yards of one another is widely different. In a group so easily 

 influenced by their surroundings, as are the Fisidia, it is not surpris- 

 ing that various forms of the same species should occur in the canal, 

 nor, in view of the traffic which passes up and down this particular 

 waterway, that these various forms should at times come to be asso- 

 ciated. An alternative explanation has been suggested by Mr. 

 Phillips, namely, that these varieties may have arisen in different 

 habitats, and by long continued habitation of these their characters 

 may have become so fixed that on reaching the canal the various 

 forms have continued to perpetuate their individual characteristics. 



The ecology of the Fisidia of the district seems to be of great 

 interest, for while all the species which Mr. Oldham has found in the 

 canal occur at most of the stations, certain common species, though 

 living in close proximity to the canal, must either be very rare in or 

 absent from it, as they have not been detected in it so far. For 

 example, F. personatum lives in ditches at Cheddington, Wilstone, 

 and Bulbourne, which are only separated from the canal by the tow- 

 ing path. In Marsworth Reservoir, which is only separated from it 

 by an embankment, two further species live, namely, F. miliufn and 

 F. obtusale ; while a fourth, F. pulchellum, occurs abundantly in a 

 tank in which logs are soaked on the bank of the canal at Rickmans- 

 worth, Herts., somewhat outside our special area. A fifth is found 

 in two habitats near Watford, Herts., also outside the limits of our 

 area; this is F. hiberniciim^ which has been taken by Mr. Oldham in 

 a stream at Cassio Bridge and in a fish-pond at Aldenham Abbey. 

 With the exception of P. personatum, which is not often found in 

 habitats such as the canal, there appears to be no reason why the 

 other four species just mentioned should not also find a place in 

 its fauna. 



This fauna as we know it to day comprises no less than eight 

 species of the genus. Of these, F. casertanum, F. nitidum, and F. 

 subtnmcahitn are widely spread throughout the British Islands and 

 occur in many kinds of habitats. F. amnicum, F. hensloitjanum, 

 F. supinum, and F. parvultim are more generally confined to streams, 

 rivers, or canals, and appear to be absent from the extreme northern 

 parts of Great Britain and from many of the western districts in 

 Ireland. The eighth species appears to be new to science and is 

 described below : P. tfmiilineatum niihi. With the curious excep- 

 tion of P. henslowanum all these species are represented in the canal 

 for the most part by very thickened and somewhat triangular varie- 



I Since the paper was read Mr. J. E. Cooper has taken specimens of P. hibernicutn and 

 P. obtusale in the canal, much to the south of our district, at Denham Lock. 



