THE DUCKBILL. 
241 
Some mammals have a summer and a winter dress. The 
hare, at the beginning of winter, doffs its summer coat for 
a suit of white. The hibernation, or winter-sleep, is a re- 
markable feature in the life of quadrupeds living in the 
north temperate zone, such as the bear, dormouse, and 
bats. During this period the temperature of their body 
falls, respiration and circulation are lowered in the one 
case, or nearly ceases in the other, and life is sustained by 
their living on the fat which accumulates on the under 
side of the neck in the so-called hibernation-glands. 
There are about 3500 species of mammals described, of 
which 2100 are living; of these 310 inhabit America north 
of Mexico. Mammals live all over the earth's surface, but 
mostly in the tropical regions. The geographical range 
of certain species is very great — for example, the cougar, 
panther, or puma ranges from British to South America 
(Chili). 
Sub- Classes of Mammals. 
1. With long toothless jaws like a duck's 
bill,layingeggs in a mammary pouch. Ornithodelpliia: Duckbill. 
2. With a pouch for holding the y ouu g . Mar supialia: Opossum. 
3. With a placenta; brain in most cases 
with convolutions, o Monodelphia: Rat, Dog, etc, 
Sub-Class I. — Ornithodelphia. 
General Characters of Monotremes. — The duckbill and 
spiny ant-eater (Fig. 284, Ecliidna liystrix) are the only 
representatives of the sub-class, of which there is but a 
single order, called Monotremes, Besides peculiarities in 
the breast-bone and other parts, one of the most obvious is 
the long, toothless jaws (there are eiglit horny teeth in the 
duckbill), which are long and narrow in the Ecliidna, or 
broad and flat in the duckbill {Ornithorliynclms para- 
doxus)^ where it is covered by a leathery integument; the 
external ear is wan tin 2^. Both animals lay true eggs. 
In the aquatic duckbill (Fig. 282) the feet ar^ webbed, 
