A knowledge of the form of the appendages to the trunk of animals is of great service to the geologist, for it has been well and truly said, that the organic remains imbedded in rocks are as medals struck in commemoration of the great revolutions which the earth has undergone, and so small a portion as the extremity of an animal is sufficient to indicate its nature, and to enable us to ascertain the class and order to which it belongs. Let us take a few examples. Some fossil bones were discovered by General Washington near his seat in Virginia. Mr. Jefferson, by whom they were examined, stated that they had belonged to an enormous carnivorous animal, which from the size of its claws he named the _Megalonyx_. Upon an examination more carefully conducted by Cuvier, it was found that in the paw the second bone of the toe was symmetrical, while in animal feeders furnished with retractile claws it is curved, and not symmetrical; the first bone, too, was the shortest, whereas in the lion and others of the feline tribe it is the longest. Whence Cuvier was led to conclude that the creature in question was a sloth of large dimensions, which fed only upon plants. Upon another occasion, from a careful consideration of the form and size of a toe found in the Palatinate, near Eppelsheim, Cuvier was able to determine that the animal to which it belonged was of the edentate tribe of _Pangolins_. It was from an examination of the hand that Cuvier decided upon the nature of the _Pterodactylus_. This curious animal partook of the nature of both the reptile and the bird. Like the bird, it was furnished with a long neck and a horny beak, but in its jaws and teeth it resembled the crocodile. It had not, however, like the bird, feathered wings without fingers to strengthen them, nor had it a wing in which the thumb alone is free, like that of the bat. Instead of the bony pieces of all the fingers being equally prolonged, the second finger only was extended to a great length, and from it the wing spread out, the other fingers being short and furnished with nails like the toes of ordinary animals. Collini, its discoverer, supposed it to be of marine origin; Soemmerring contended that it was a mammal; but its true place in the animal series was first assigned to it by Cuvier, who has satisfactorily shewn that it is in fact intermediate between a bird and a reptile.