132 THE CHARACTERS OF GENERA. 
be singled out, named, and entered in our sys 
tems, without being duly characterized, and it is 
much to be lamented that there is no possibility 
of checking the loose work of this kind with which 
the annals of our science are daily flooded. 
It would, of course, be quite inappropriate to 
present here any general revision of these groups ; 
but I may present a few instances to illustrate 
the principle of their classification, and to show 
on what characters they are properly based. 
Among Reptiles, we find, for instance, that the 
Genera of our fresh-water Turtles differ from 
each other in the cut of their bill, in the arrange- 
ment of their scales, in the form of their claws, 
etc. Among Fishes, the different Genera includ- 
ed under the Family of Perches are distinguished 
by the arrangement of their teeth, by the serra- 
tures of their gill-covers and of the arch to 
which the pectoral fins are attached, by the na- 
ture and combination of the rays of their fins, 
by the structure of their scales, etc. Among In- 
sects, the various Genera of the Butterflies differ 
in the combination of the little rods which sugtain 
their wings, in the form and structure of their 
antenns, of their feet, of the minute scales 
which cover their wings, etc. Among Crustacea, 
the Genera of Shrimps vary in the form of the 
claws, in the structure of the parts of the mouth, 
in the articulations of their feelers, etc. Among 
