enjoy the most numerous faculties, the most delicate sensations, the most varied powers of motion. Their organization is more differentiated than in any other Vertebrates, and they have a more perfect combination of powers. They are inferior to Birds in muscular movement, their respiration being less in amount, and the circulation less rapid, on which account their demand for food is not so constant.
The generality of them are terrestrial; some inhabit trees, others burrow in the ground, whilst a few can fly, and some are perfectly aquatic. With such differences in habit we of course find corresponding differences in external structure. The anterior extremities in Bats are lengthened to support the flying membrane, whilst in Whales they are shortened and fin-like, and the terminal points of the phalanges vary from sharp raptorial claws, to the solid hoof of the Horse, or the flat pad of the Camel and Elephant. On these characters, combined with those of the teeth, are founded the different orders of Mammals.
The form of the body varies, but we can generally distinguish the head, neck, and trunk, and most have caudal appendages. The head varies greatly in its form and proportions, as does the ear, and from these also, characters of more or less importance are drawn; but the teeth, in form, number, differences, and relative position, afford the most varied, prominent, and decisive characters, as well for the Orders of Mammals as for genera, and even sometimes species, and require more lengthened notice than any other point in the external anatomy of Mammals.
Teeth being used by Mammals both to seize and collect food, and to reduce it to a fit state for swallowing, their form furnishes a clue to the instincts and habits of the animal. They are placed in a single series along the edge of the upper and lower jawbones, so as to oppose each other, and are always fixed in cavities called sockets or alveoli, an arrangement which elsewhere is only found in the Crocodiles among Reptiles.
Ivory, or Dentine, forms the sole material of some simple teeth, as in some Cetacea, and in the tusk of the Elephant; but in most teeth another substance, crystalline in texture and extremely hard,
