ORDER IV.—_Loricata._[9] This corselet or armor-covered family includes Crocodiles and their allied species. Their upper parts are covered with a corselet of bony plates set in the tough, leathery skin. The jaws are long, and have many strong, conical teeth, fixed in sockets. The lower jaw extends back of the cranium, and the upper is hinged and movable. Crocodiles belong in tropical climates, are sluggish animals, and live a long time if not destroyed by violence. Their legs are short but powerful, and when on land they manage to wriggle, or drag, their immense bodies along with considerable speed. They are found in India, and in all the large rivers of Africa. The Gavial of the Ganges, the Crocodile, and the Mississippi Alligator are closely related, though there are some structural differences. Of whatever variety, whether of the Old World or of the New, they are more numerous than desirable, not being noted for either their beauty or usefulness. CLASS III.—_Aves._[10] Birds furnish a delightful study, and their leading characteristics are easily stated. Widely as they differ, they all have a common type of structure, and are essentially alike in those particulars that distinguish them from other orders in the animal kingdom. They are all warm blooded, feathered, biped vertebrates, mostly with wings fitted for flight, and with either webbed feet for swimming, or claws for seizing, scratching, and perching. [Illustration: ANATOMY OF A HEN. _Ex._—A, cranium; B, cervical vertebræ; G, furcula, or merry thought; J, sternum, or breast bone; H, cloaca, surrounded by the pelvic bones; E, caudal vertebræ, terminating in plowshare bone; D, lumbar vertebræ, or rump; C, F, dorsal vertebræ, to which the ribs are attached.]