Class. MAMMALIA. 



GENERAL INTRODUCTION. 



Mammals are the most highly organized forms of vertebrate 

 animals, and may conveniently be defined as follows : 



Warm-blooded animals, having the heart divided into four 

 cavities, two auricles and two ventricles, and with a complete 

 double circulation ; having the lungs separated from the abdomen 

 by a complete muscular partition, termed the diaphragm ; having 

 the skin, more or less clothed with hair in its different modifi- 

 cations ; and with the young produced alive (except in the case 

 of the very few aberrant forms, which constitute the Monotremes 

 or PROTOTHERIA, see p. 1), and nourished for some time after 

 birth by means of the mammary glands with which the female is 

 provided. 



Limbs. -In the majority of Mammals the two pairs of limbs are 

 well developed, and specially adapted for progression on the surface 

 of the earth, but they are, in many cases modified to suit the re- 

 quirements of the particular mode of life to which each individual 

 genus has become habituated. For instance : in some the fore 

 limbs are specially formed for burrowing, as in the Wombat and 

 Mole ; in others for climbing, as in the Opossums and Monkeys ; 

 in others for flying, as in the Bats ; and in others again for 

 swimming, as in the Seals and Whales, in which case the hind 

 limbs are rudimentary or more often entirely suppressed. 



Tail. Most Mammals are provided with a tail, which, however, 

 may be rudimentary, and therefore functionless, as is the case 

 with the Koala, the Ape, the Deer, and many other genera ; 

 prehensile, or formed for grasping, as with the American Opossums, 

 the Cuscus, and certain families of Monkeys ; or fluke-like and so 

 formed for rapid motion through water, as with the Whales and 

 Dolphins. 



TEGUMENTARY STRUCTURES. 



Epidermis. Almost all Mammals are clothed, and in the 

 majority of cases thickly clothed, with a peculiarly modified 

 form of the outer skin, or epiderm, variously known as hair, fur, 

 wool, etc., which has its root at the bottom of a cavity in the 

 derm or true skin. This substance assumes various forms, and 

 is of various sizes and degrees of rigidity, from the soft fur of 

 our Flying Squirrels to the spines of the Echidnas. The obvious 

 purpose of this covering is to protect the skin against external 

 influences, such as cold and damp, and in some cases against the 



