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 
