PISIDIUM. 17 



lusks and of their shells; and it has received almost 

 the universal assent of conchologists. The habits of the 

 little Pea shells are the same as those of the larger 

 members of the same family ; and they inhabit nearly 

 the same situations. One species (P. pusillmn) does not 

 seem to require a constant, or even a frequent supply of 

 water, being often found living at the roots of bog-moss 

 vs^hich is dried up in the summer, and of grass in mea- 

 dows which are only irrigated in the spring ; and Nils- 

 son noticed, in his history of the land and river mol- 

 lusks of Sweden, that he had frequently found the same 

 species (which he erroneously referred to the P. fontinale 

 of C. PfeifFer) living between the bark and wood of fallen 

 trees in moist places. They possess the same faculty as 

 the Sphceria, of floating, or creeping in an inverted posi- 

 tion under the surface of the water. These tiny Pea 

 shells, or cockles, swarm in every slow river, streamlet, 

 lake, pool, horse-pond, ditch, and open drain ; and they 

 are greedily devoured by fish and ducks. In their turn, 

 they are fond of animal food ; and Dr. Baudon, in his 

 admirable Monograph, says that he has often observed 

 Pisidia attached to the drowned carcases of small ani- 

 mals, as well as to bones which had been thrown into 

 ditches and streams, and from which the muscular fibres 

 had not been removed. Perhaps, however, animalcula 

 fed upon the meat, and were the real objects of attrac- 

 tion to the Pisidia. Their shells are sometimes so 

 thickly encrusted with a ferruginous or mineral deposit 

 from the muddy sediment of the water which they in- 

 habit as to resemble small lumps of dirt. This deposit 

 appears to be partly owing to a secretion of the animal, 

 aided by its generally inactive habits. Water-beetles do 

 not allow themselves to be clogged in the same manner. 

 Whether this is one of the artifices by which animals 



