18 SPH^BIID^E. 



escape the observation of their natural enemies is a 

 question which requires a molluscan mind to solve. 



The critical investigation of the different species which 

 compose this genus is quite as difficult as it is with regard 

 to the large freshwater mussels. Little reliance can be 

 placed on the characters afforded by an examination of 

 the body, or soft parts, of the animal. The form and 

 comparative length of the tube are especially liable to 

 vary even in the same individual ; and under the influ- 

 ence of heat and light the most Protean changes with 

 respect to this organ may be observed. The size of the 

 foot is equally a deceptive character ; and colour is al- 

 ways a most uncertain test. The general shape and 

 appearance of the shells, as well as the position of their 

 beaks, appear to offer almost the only reliable grounds 

 of distinction. Size, substance, sculpture, and lustre are 

 not of much account, as they mainly depend on the 

 chemical ingredients of the water inhabited by these 

 mollusks, as well as on their supply of food. In making 

 an investigation like the present, there appear to be four 

 courses open to the naturalist. The first, which is, 

 perhaps, the easiest, is to reduce all hitherto described 

 species to one or two, and thus to cut the Gordian knot 

 without further ceremony. The second, which has been 

 pursued to such an extent on the Continent and in the 

 United States of America, is to multiply the number of 

 species ad infinitum. The only check which can be im- 

 posed on this method of wholesale and indiscriminate 

 fabrication is the bar of scientific opinion ; and in coun- 

 tries where nearly all the naturalists are culprits, there 

 is not much likelihood of justice being so severely admi- 

 nistered as to prevent the repetition of such venial 

 offences. The third course is, to adopt the labours of 

 preceding writers without any inquiry. And the fourth 



