STIU.KOX : I'ISIDIUM FAUNA OK IIIE CRAND JUNCTION CANAL. 295 



ened, even more tumid and the hinge more arched ; and the hgament 

 pit still shorter. 



Habitat : — In the Grand Junction Canal at Cowroast Lock, Duds- 

 well, Herts. It is also found in the Thames, and in the Barrow, 

 Nore, and Suir in Ireland, and is no doubt to be found in most of 

 the larger river basins where the water contains a sufficient quantity 

 of calcareous matter. The old theory that these thick-shelled forms 

 of the Pisidia are produced by running water appears to be a myth. 

 This variety may be distinguished from P. supinum, for which it 

 would appear to have been mistaken more than once, by its shorter 

 and broader ligament pit, its broader umbones, much closer and 

 more irregularly placed strire, as well as by the fact that it is never 

 appendiculate. 

 P. nitidum Jenyns.^ Plate 7, figs. 5-13. 



A puzzling series of forms of this shell occurs in the canal, the 

 most thickened one of which I was unable to identify at first owing 

 to its more or less globular shape, small umbones, and very coarse 

 striation. In the first and second character it resembles P. hiber- 

 nicni/i, while in the third it approaches P. pidchelluin. More or less 

 typical shells. are also found in the canal, though these are rare, and 

 various forms intermediate between them and the thickened one 

 mentioned above also occur, so that I now feel justified in regarding 

 the latter as the thick-shelled representative of this species and giving 

 it the title crassa. 



var. crassa nov. Plate 7, figs. 5-S. 



Smaller than the type, more globular,'^ thicker, hinge more arched, 

 and the lateral teeth much shorter and stouter, strise generally more 

 strongly marked, so that the usually distinct grooves across the um- 

 bones are not very noticeable; umbones smaller and more prominent; 

 siphon similar. 



Habitat : — In the Grand Junction Canal, near Marsworth Church, 

 Bucks. A similar form is abundant in the Thames, but the shells 

 from that river are not as a rule so strongly striate. A specimen 

 referable to this variety, though not quite typical of it, taken by Mr. 

 Oldham in the canal at Marsworth in 1906, is figured in Mr. Wood- 

 ward's Catalogue — plate xxv., fig. lob — as a form of/', supinum. The 

 remainder of the gathering is in Mr. Oldham's collection labelled in 

 Mr. Woodward's writing: ''• P. supinum (ovate form)." The var. 

 crassa may be distinguished from P. supinum by its more equilateral 

 hinge and outline, smaller cardinal teeth, very short and broad liga- 



1 See note added in press on page ^oj. 



2 This description might be thought to fit JeflVeys's /'. nitidum var. globosa, but that shell 

 is really a form of/". obtusaU, as pointed out by Mr. Woodward (Catalogue, page ii). 



