STKT.rOX : PISiniUM P-AUNA OK 'I'lFK CRANH jUNCTinN' CANAT,. 30'; 



against deciding hastily on any of these small bivalves, withojit seeing 

 f/ie living atiimalT Part of this sentence is quoted by Mr. Woodward 

 (Cat. p. to) but since he omits the essential portion, which I have 

 placed in italics, he would appear to have missed the whole signi- 

 ficance of Jenyns' remark. 



Description of Plates. 



To judge by the plates in Mr. Woodward's Catalogue, photography 



has failed so f;ir as reproducing recognisable representations of the 



hinges of the Pisidia. The figures on the accompanying plates are 



based on outline drawings by camera lucida, afterwards shaded. The 



author hopes that they present the hinges of the various species as they 



appear to him and that they will convey to others a better idea of the 



shells than the illustrations in the older works. The siphons are 



approximately to scale. 



Plate 7. 



Figs. I, 2, 3, 4. Interior and exterior views of the thickened form 

 of P. subtrmtcatum from the canal at Wilstone. The short curved 

 cardinal teeth in this form differ markedly from those of normal 

 specimens. 



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

 nitidum var. crassa from the canal near Marsworth Church. 



Figs. 9, 10, II, 12, 13. Various forms assumed by the siphon of 

 var. crassa : fig. 10 shows the siphon of a very old specimen unfolding : 

 fig. 12 a siphon without a distinctly crenulated margin. 



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

 example of P. siipinuin from the canal at Marsworth. 



Fig. 18. The siphon of a P. supinuin from Marsworth. 



Figs. 19, 20, 21, 22. Interior and exterior views of a very large 

 specimen of P. henslowanwn from tlie canal at Cowroast Lock, 

 Dudswell. 



Fig. 23. The siphon of a P. henslowanum from Marsworth. 



Figs. 24, 25. Interior views of an old specimen of P. casertanum 

 var. hiimeriformis from the canal at Marsworth. 



Figs. 26, 27, 28, 29, 30. Variations in the shape of the siphon 

 assumed by examples of var. hunierifomnis and the more typical form 

 of P. casertanum from the canal at Marsworth. 



Figs. 31, 2^2, 33, 34. Interior and exterior views of a specimen of 

 P. casertanum var. ponderosa from the canal at Cowroast Lock, 

 Dudswell. 



Figs. 35, 36, 37, 38. Interior and exterior views of a specimen of 

 P. amnicum from the canal near Hospital Farm, Marsworth. To a 

 reduced scale. 



i' ig- 39- i'he siphon of a P. amnicwn from Marsworth. 



