294 JOURNAL Ol-' CONCHOLOGY, VOL. 15, NO. 10, SEPTEMBER, I918. 



Phillips suggests, they may be distinct races derived from different 

 sources and breeding true in the canal. Additional material and a 

 study of living specimens has convinced me that all these varieties 

 are merely well-marked phases of this plastic species. Exteriorly, ail 

 are similarly sculptured, the striation being very fine (close) and some- 

 what irregular, and although there is a good deal of variation in the 

 siphons, this variation was found to bear no relation to the form of 

 the shells. All the forms are more or less thickened, but as they 

 approach the triangular shape the thickening becomes more pro- 

 nounced. Continental authorities appear not only to have created 

 numerous species out of the multiple forms of P. casertatium, but to 

 have ascribed some of its varieties to other well-known and perfectly 

 distinct species, and so increased the chaos in which the whole group 

 is involved. The form in the canal which most nearly approaches 

 the type, as figured by Mr. Woodward, is rare. It is somewhat 

 similar to that found in Marsworth Reservoir — plate 9, figs. 29-30 — 

 but is a little more oval. The smaller, shouldered form, which is 

 common — plate 7, figs. 24-25 — is very similar to shells taken by Mr. 

 Phillips in Lough Rea, Co. Galway, which have been identified and 

 recorded by Mr. Woodward in his Catalogue as P. steenbuchii 

 (Moller). The third phasis of this shell which lives in the canal— 

 my var. ponderosa — plate 7, figs. 31-34 — may prove to be the sup- 

 posed inappendiculate form of P. supitmm mentioned by the describer 

 of that species, Adolph Schmidt, Clessin and other authors. Although 

 the multiplication of varietal names has many objections, I feel that 

 there is no alternative but to provide names for the above very 

 definite stages in the variation of P. casertatrnvi, the origin of which, 

 I think, will be traced eventually to their early environment. 



var. humeriformis nov. Plate 7, figs. 24-25. 

 Smaller than the type, more tumid, more squarrose, the dorsal out- 

 line having a distinctly shouldered appearance ; hinge similar to that 

 of the type but rather shorter and more arched owing to the shape of 

 the shell ; ligament pit shorter and broader than that of the type for 

 the same reason [humeriforme on pi. 7 by error]. 



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

 Bucks. This variety occurs also in the Thames, and in some rivers 

 and lakes in Ireland. It seems to live in water in which there is not 

 much decaying vegetable matter. As a general rule shells of this 

 variety are thick and heavy, but thin-shelled forms with a similar 

 outline, though less tumid, occur in certain districts. 



var. ponderosa nov. Plate 7, figs. 31-34. 

 Larger than the last variety, more triangular, and very much thick- 



