CYCLAS. 255 



tenuated and produced, Avhile in the British speci- 

 mens this part of the shell is regularly rounded ; 

 but I have no means of ascertaining whether this is 

 the usual state of the species, or only an accidental 

 variation in the individual we possess, and one can 

 scarcely believe, without better proof, that the same 

 species should be found in Europe and the United 

 States ; therefore I shall for the present consider it 

 as distinct. 



The species is intermediate in size and form be- 

 tween Cyclas rivicola and C. cornea. 



117. 4. CYChAS pisidioides. — Shell ovate, subtrigonal, 

 involucres olive pale-edged, slightly concen- 

 trically wrinkled, rather rounded in front, 

 somewhat produced, with a broad subangular 

 slope behind ; the umbones subanterior, re- 

 gularly convex. Siphons united nearly to 

 the end, the upper shorter, subconic; aper- 

 tures circular, simple, the lower rather larger, 

 about twice the length of the upper when 

 expanded, cylindrical ; the opening circular, 

 simple. 



Sphffii'iura pisidioides. Gray, Aim. and Mag. N. H. xviii. 

 25. 1856. 



Inhab. Paddington Canal. 



The adult shells are 6 lines long, 5 high, and 4 

 thick. They have much the appearance of a large 

 swollen Pisidium, but have the two distinct siphons 

 of the genus Cyclas. 



The young shells which were deposited In the 

 glass of water during the night were much com- 

 pressed and nearly regularly oblong. They varied iu 



