about to consider, while in none do they manifest any near affinity to true Fishes. We are therefore constrained to designate the Class by a term founded on the last of the distinctive peculiarities enumerated above.
The sphere of action assigned to the Mammalia is the solid earth, on which they walk or run with various degrees of agility; some, however, burrow beneath its surface, as the Mole; others by a very interesting modification of the ordinary structure, emulate the rapid and continued flight of birds, as the Bats; while yet others, mostly of great bulk, inhabit the ocean, as the Seals and Walruses, the aquatic Pachydermata, and the Cetacea.
Certain important peculiarities of organization require the separation of the Mammalia into two divisions of very unequal extent, which are named Placentalia and Marsupialia. The details of these peculiarities would be unsuitable to the character of the present treatise; the most obvious is the presence of a remarkable pouch on the abdomen of the female in the Marsupialia, in which the teats are situated, and into which the young is transferred, at a very incipient stage of development, and there nourished till it is fully formed and its powers are somewhat matured.
