36 
GENERIC SYSTEM. 
composed of more than two pieces ; Bivalves, of 
turn 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- 
