304 JOURNAL OF CONCHOLOGY, VOL. I5, NO. ID, SEPTEMBER, I918. 



Plate 8. 



Fig. I. The siphon of a P. teniiilineatinn from Marsworth ; a 

 front view, b side view. 



Fig. 2. The siphon of a P. parvulum from Marsworth. 



Fig. 3. The siphon of a very young P. siipinum from Marsworth. 



Figs. 4, 5. Interior views of a very large specimen of P. tenui- 

 lineaticm from the canal at Rothersthorpe, Northants. In the left 

 valve of this specimen the anterior ridge of c. 2 does not cross bu-t 

 joins that of a. II. 



Figs, 6, 7, 8, 9. Interior and exterior view^s of an abnormal speci- 

 men of P. temiilineatum from the canal near Marsworth Church, 

 having p. Ill in the left valve. 



Figs. 10, II, 12. 13. Interior and exterior views of a typical speci- 

 men of P. tenuilineatum from the canal at Marsworth. 



Figs. 14, 15, 16, 17. Interior and exterior views of a specimen of 

 P. parvtdum from the canal at Marsworth. 



Figs. 18, 19, 20, 21. Interior and exterior views of a specimen of the 

 large thickened form of P. parvuhan from the canal at Berkhamsted. 



Figs. 22, 23, 24, 25. Interior and exterior views of a very juvenile 

 specimen of P. supimmi from the canal at Wilstone. 



Plate 9. 



Figs. I, 2, 3, 4. Interior and exterior views of a specimen of a some- 

 what thin form of P. hibernicum from a fish-pond at Aldenham Abbey. 



Figs. 5, 6, 7, 8. Interior and exterior views of a specimen of P. 

 milium from Marsworth Reservoir. 



Figs. 9, 10, II, 12. Interior and exterior views of a specimen of 

 P. obtusale from Marsworth Reservoir. Fig. 9 shows j;^. ///curving 

 across p. I towards the umbo and ending in a kind of lump or 

 " pseudo-callus." The pseudo-callus is perhaps the easiest way to 

 separate P. obtusale from the forms of P. hiberiiicum and other species 

 which at times resemble it. 



Figs. 13, 14, 15, 16. Interior and exterior views of a specimen of 

 P. pulchellum from a tank at Rickmansworth. 



Figs. 17, 18, 19, 20. Interior and exterior views of a specimen 

 from Boveney, Bucks., identified and recorded by Mr. Woodward as 

 "Z'. steenbiichii (Moller)." 



Figs. 21, 22. Interior views of a specimen of P. nitiduiii from 

 Marsworth Reservoir. 



Figs, 23, 24, 25, 26. Interior and exterior views of a specimen of/". 

 personatum {^P. pusilliun Jenyns) from a ditch by the canal at Wilstone. 



Figs. 27, 28. Interior view of the form oi P. casertatmm which is 

 associated with P. personatum at Wilstone. 



Figs. 29, 30. Interior view of a specimen oi P. casertanum asso- 

 ciated with P. milium and other species in Marsworth Reservoir. 



