3. _The Amphibia_, consisting of the seals, and other allied animals. V. _Marsupialia_, including the opossums and the kangaroos. VI. _Monothrema_, containing the Echidna and Ornithorhynchus of Australia. VII. _Rodentia_, or gnawing animals. The principal of these are the squirrel family, mice and rats, hares and rabbits, the beaver, the porcupine, and the guinea-pig. VIII. _Edentata_, or toothless animals, that is, without front teeth. The principal of these are the sloths, the armadillos, and the ant-eaters. SECTION II.--_Ungulated or Hoofed Mammalia._ IX. _Pachydermata_, or thick-skinned animals. The principal of these are the elephant, the hippopotamus, the rhinoceros; the horse family, including the ass, the mule, the zebra, and the quagga; the wild boar family, and the tapir. X. _Ruminantia_, or ruminating animals, the principal of which are the camel family, the deer family, the giraffe, the antelope family, the goat family, the sheep family, and the ox family. SECTION III.--_Aquatic Mammalia, having no Hind Limbs, and the Fore Limbs converted into Fins._ XI. _Cetacea_, or sea mammalia, the principal of which are the whale family, the dolphin family, the manati, the porpoise family, and the narwhal, or sea-unicorn. THE AVES, OR BIRDS, Lay eggs from which their young are hatched by what is called incubation. Their skins are covered with feathers; and their jaws are horny, without teeth. Their blood is warm, and circulates like that of the mammalia. The six orders of Aves are as follow:-- 1. _Raptores_, or birds of prey. These birds are distinguished by a very strong and sharp bill more or less curved, but always hooked at the extremity of the upper mandible, which is covered at the base with a kind of skin called the cere. The nostrils are usually open. The legs are very strong, the feet are large, and the toes, which are four in number, are armed with very strong, sharp, curved claws. The principal raptorial birds are the vultures, including the condor; the falcon family, including the eagles, hawks, kites, and buzzards; and the owls.