CYCLAS. 
255 
tenuated and produced, while 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. 'pisidioides. — -Shell ovate, sub trigonal, 
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. 
Sphaerium pisidioides. Gray^ Ann, 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 in 
