OBSERVATIONS ON MOLLUSKS. 



SMALL FRESH-WATER BIVALVES.* 



SOME years ago, when studying these little ani- 

 mals,-)- I was greatly struck with the differences of 

 habit observable in species, which externally so much 

 resembled each other as hardly to offer, at least to 

 an unpractised eye, any distinguishing characters. J 

 These bivalves are common in ponds and ditches, as 

 well as in streams ; the smaller species are not unfre- 

 quently found in the gullies cut in the pastures for 

 the purpose of draining the soil. They all live 

 readily in confinement for several days when kept in 

 water ; a.nd under these circumstances their different 

 habits may be conveniently observed. Occasionally 

 it will be found that they become languid and inert, 

 especially if they have been confined a long time, 

 but they may generally be roused into activity by 



* Cyclada of Gray's Manual, p. 277. 



t See my " Monograph on the British species of Cyclas and 

 Pisidium," Camb. Phil. Trans, vol. iv. p. 289. 



Probably in all cases in which species are really distinct, 

 except, perhaps, in the lowest and most imperfectly organized 

 animals, there exist some corresponding differences of habit, as 

 well as of structure, to mark the distinction. These differences 

 cannot be too closely inquired into by the naturalist. When- 

 ever it is in his power to watch the habits of any supposed new 

 species, either in its native haunts, or by keeping it in confine- 

 ment, let him not pronounce positively respecting it, till he has 

 called to his aid the valuable assistance to be derived from this 

 source. The importance of rioting the smallest fact in respect of 

 the habits of animals has been already spoken of in the Introduc- 

 tion to this work (10). 



