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 sustain 

 their wings, in the form and structure of their 

 antennae, 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 



