300 JOURNAL OF CONCHOLOGY, VOL. I5, NO. lO, SEPTEMBER, I918. 



that gives much food for thought. It occurs everywhere, and is con- 

 stantly associated with the heavy-shelled forms of the other species. - 



Appendix. 



Containing a list of the species of Pisidia which occur in the dis- 

 trict through which the canal runs, but which have not yet been found 

 living in it. 

 P. personatum (Malm) B. B. Woodward. Plate 9, figs. 23-26. 



Is frequent in the district, and lives right up to the edge of the 

 canal in drains and ditches. 



I believe this to be the P. pusillum of Jenyns, and most British 

 authors,'^ though they confounded forms of P. casertanuvi with it, and 

 perhaps other species. Clessin (3) and Westerlund (2) refer to 

 lA2\'a\^% personatum as a variety of P. obtusale, and it may be remarked 

 that Malm's diagnosis cannot be said to fit the species here intended. 

 P. obtusale (Lamarck) Jenyns. Plate 9, figs. 9-12. 



Plentiful in Marsworth Reservoir, associated with P. casertamim, 

 subtruncahi?n, miliiivi, and nitidum. It is also frequent throughout 

 the district in ponds, etc. 

 P. milium Held. Plate 9, figs. 5-8. 



Common in Marsworth Reservoir and frequent in the district. 

 P. pulchellum Jenyns. Plate 9, figs. 13-16. 



The nearest stations discovered by Mr. Oldham for this species are 

 the tank at Rickmansworth, mentioned in the first part of the paper, 

 and a fish-pond at Aldenham Abbey, both habitats being in Herts. 

 P. hibernicum Westerlund. Plate 9, figs. 1-4. 



A thin form has been taken by Mr. Oldham in a brook bordering the 

 towing path at Cassio Bridge near Watford, and again in a fish pond 

 at Aldenham Abbey, Herts., both stations being outside the area 

 dealt with in the paper. Mr. Oldham informs me that he considers 

 the shells of Jenyns's P. obtusale var. /3 preserved at Bath to belong to 

 the present species and it is interesting to think that Jenyns recognised 

 its distinctness sufficiently to refer it to a variety of one of his species. 

 It is not a rare shell and is generally associated with P. nitidum, 

 subtruncaium and milium, and sometimes with other species. It may 

 be distinguished from the associated specimens of its nearest ally, 

 P. nitidum, by its smaller and more prominent umbones, delicate 

 though well-marked striation, its narrow tube-like siphon and its 

 longer cardinal teeth ; especially by its c. 2 which is sometimes twice 

 as long as that of the associated P. nitidum. In most of the low- 

 land forms of the species the lateral teeth are not greatly twisted, 



1 For a possible explanation of this remarkable fact see footnote on page 293 supra. 



2 See 5, in bibliography. Also note page 302 below. 



