36 GENERIC SYSTEM. 



composed of more than two pieces ; Bivalves, of 

 tiro distinct parts; and Univalves, of one entire 

 formation. 



Linnaeus places them in the above order : and 

 although it may, at first sight, appear more syste- 

 matical to begin with the Univalves, and proceed 

 to those of more complicated structure ; still, upon 

 due investigation, the Linnaean series will, at least, 

 by many, be found most eligible. On this subject 

 there has been, and probably will be, much differ- 

 ence of opinion ; yet it is to be considered, that not 

 only the number, but the beauty and importance of 

 the species increase progressively from the Multi- 

 valve division, which contains the fewest, to the 

 Univalve, which far exceeds the other two collec- 

 tively. Much more discrimination and experience 

 are requisite, to point out the varieties, which 

 amount to some hundreds, of Patella, than to ar- 

 range the comparatively small genus Chiton. It 

 is not the number of valves, but the number of the 

 different specimens to be examined, which consti- 

 tutes the difficulty of arranging a genus, and of 

 defining its limits with perspicuous accuracy. 



The strong affinities existing between some sub- 



