~ A Monograph of the Species of Pisidium. 349 
The Rev. Mr. Jenyns of England, in his Monograph of the 
British Cyclas and Pisidium, accompanied not only by magni- 
fied figures of the shells, but also of the animals, was the fitst 
to place the subject in its true light. 
As yet this genus has been but little or not at all acknow- 
ledged in this country. In Europe, Des Hayes himself in 
his notes to La Marck's Invertebrata, does not consider it 
as sufficiently distinct from Cyclas to constitute a genus; in 
his work, however, on Conchology applied to Geology, he 
has not only given it full justice but has thoroughly worked 
it up. 
Some naturalists, and especially Gray, persist in applying 
to it the name of Pisum Megerle. Notwithstanding that, 
Megerle's type of the genus Pisum, the C. rivicola, is a genu- 
ine Cyclas. 
CHARACTERS. 
Animal. Lobes of the mantle without tentacles, united 
posteriorly into a short, single, syphonal tube. Oral aperture 
small, tentacles of the mouth triangular, elongated. Gills of 
medium size, unequal, united behind the foot. Foot small, 
triangular, capable of great extension. 
hell. Small, beaks terminal, suboval. Cardinal teeth 
terminal, small, two in the left valve, at times united into 
one. Lateral teeth longitudinal, compressed, lamelliform, 
double in the right valve. 2 
The differences between Pisidium and Cyclas are very ma- 
terial both in the animal and in the sbell. As may be seen 
by the above description, the animal of Pisidium has the 
edges of the mantle united into one single syphonal tube, 
whereas in Cyclas the edges of the mantle are united into one 
syphonal tube, also single at its base, but double at the extre- 
Mities, 
“The foot of Pisidium is widely different from that of 
Cyclas; it is somewhat similar to that of Cardium. This 
_ Organ is capable of great extension, and when prolonged to 
