GENERIC SYSTEM. 51 



Linnaeus places them in the above order : and 

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

 stematical to begin with the Univalves, and pro- 

 ceed 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 

 difference of opinion ; yet it is to be considered, 

 that not only the number, but the beauty and im- 

 portance of the species increase progressively from 

 the Multivalve division, which contains the fewest^ 

 to the Univalve, which far exceeds the other two 

 collectively. Much more discrimination and ex- 

 perience are requisite, to point out the varieties, 

 which amount to some hundreds, of Patella, than 

 to arrange the comparatively small genus Chiton. 

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

 the different specimens to be examined, which 

 constitutes the difficulty of arranging a genus, and 

 of defining its limits with perspicuous accuracy. 



The strong affinities existing between some sub- 

 divisions of genera, which, according to prescribed 

 rules, are nevertheless distinct, and the fallibility 

 s2 



