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 arrangement of their scales, in the form of their claws, etc. Among Fishes, the different Genera included under the Family of Perches are distinguished by the arrangement of their teeth, by the serratures of their gill-covers, and of the arch to which the pectoral fins are attached, by the nature and combination of the rays of their fins, by the structure of their scales, etc. Among Insects, 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 Worms, the different Genera of the Leech Family are combined upon the form of the disks by which they attach themselves, upon the number and arrangement of their eyes, upon the structure of the hard parts with which the mouth is armed, etc. Among Cephalopods, the Family of Squids contains several Genera distinguished by the structure of the solid shield within the skin of the back, by the form and connection of their fins, by the structure of the suckers with which their arms are provided, by the form of their beak, etc. In every Class, we find throughout the Animal Kingdom that there is no sound basis for the discrimination of Genera except the details of their structure; but in order to define them accurately an extensive comparison of them is indispensable, and in characterizing them only such features should be enumerated as are truly generic; whereas in the present superficial method of describing them, features are frequently introduced which belong not only to the whole Family, but even to the whole Class which includes them. X.