FRESH-WATER BIVALVES. 325 



the sudden application of cold spring-water ; and this 

 is by far the best method of getting a sight of the 

 siphonal tubes which in some cases afford good dis- 

 tinguishing characters. Certain species, however, 

 are at all times much more active than others. Thus 

 the Pisidium obtusale * is almost always in motion, 

 residing less at the bottom of the vessel in which it 

 is kept than any other species in the family : it trans- 

 ports itself rapidly along from place to place, and 

 appears to delight in floating masses of confervae and 

 other weeds. The P. amnicum^ on the contrary, 

 keeps wholly at the bottom, with the anterior half of 

 its shell buried in the mud. The P. pusillum J ap- 

 pears to be somewhat amphibious in its habits : it fre- 

 quently leaves the bottom of the vessel, and, ascend- 

 ing the sides, takes up its residence immediately 

 above the edge of the water, with its shell wholly 

 exposed : there it will remain tranquil for a length 

 of time. The most remarkable habit, however, pos- 

 sessed by some species of these bivalves is that of 

 walking, if we may so express it, on the under side 

 of the surface of the water. This habit is common 

 to many gasteropoda, but I am not aware of its 

 having been observed in any of the bivalve mollusks. 

 The action consists in the animal extending its foot 

 along the surface, with its shell immersed, and in an 

 inverted position. In this manner it contrives to 

 traverse the water as though it were crawling along a 

 solid plane. This habit is not exercised, except by 



* Monograph, p. 301, tab. xx. f. 13. 



t Id. p. 309, tab. xix. f. 2. % Id. p. 302, tab. xx. f. 46. 



