MAMMALIA.
Mammals are warm-blooded Vertebrate animals that nourish their young with milk secreted by the females in glands situated in pairs on the under surface of the body. All, with a very few exceptions (chiefly Cetacea), are covered with hair. The great majority possess teeth, and the higher forms are heterodont, or furnished with teeth of different kinds, and diphyodont, or bearing two sets—the first, known as milk or deciduous teeth, generally coming into use at birth or soon after, and being subsequently replaced by a second or permanent set. Most mammals possess two pairs of limbs like other normal vertebrates, and the terminal extremities of these limbs, with but few exceptions, are furnished with nails, claws, or hoofs. The thoracic cavity, containing the lungs, is completely separated by the diaphragm from the abdomen.
The class Mammalia is divided into the following subclasses:—
| A. |
Oviparous, both genito-urinary passage and anus opening into a cloaca.
|
| I. |
PROTOTHERIA, Ornithodelphia or Monotremata.
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| B. |
Viviparous, genito-urinary orifice external and distinct from anal[1].
|
| a. |
No allantoid placenta[2].
|
| II. |
METATHERIA, Didelphia or Marsupialia.
|
| b. |
An allantoid placenta.
|
| III. |
EUTHERIA, Monodelphia or Placentalia.
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Of the subclasses the Prototheria or Monotremata are peculiar to the Australian region, whilst the Metatheria or Marsupialia are only found in the same region and in America (chiefly in South America). The Eutheria or Placentalia comprise, according to Professor Flower's latest classification, nine orders, all represented in India. These orders may be distinguished (so far, at all events, as Indian genera are concerned) by the characters shown in the
- ↑ The two open on a common outlet in some genera of Insectivora.
- ↑ For full details as to the significance of these characters in classification, consult Huxley's 'Introduction to the Classification of Animals,' p. 87, or Balfour's 'Comparative Embryology,' vol. ii. p. 176, or 'Encyclopædia Britannica,' article "Mammalia," pp. 369, 371, &c.
