MAMMALIA. 



Mammals a,re 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 pos- 

 sess teeth, and the higlier 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 gcnito-urinary passage and anus opening into 



a cloaca. 



I. PROTOTHERIA, Ornithodelphia or Monotremata. 



B. Viviparous, genito-urinary orifice external and distinct from 



anal *. 



a. No allantoid placenta f. 



II. METATHERIA, Didelphia or Marsupialia. 



b. An allantoid placenta. 



III. EUTHERIA, Monodelphia or Placentalia. 



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 Etitheria 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 Insect ivwa. 



t For full details as to the significance of these characters in classification, 

 consult Huxley's ' Introduction to the Classification of Animals,' p. 87, or Bal- 

 four's ' Comparative Embryology,' vol. ii. p. 176, or ' Eucyclopiedia Britannica,' 

 article "Mammalia," pp. 369, 371, &c. 



B 



