THE DUCKBILL. 241 



Some mammals haye 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 neclc in the so-called hibernation-glands. 



There are about 3-500 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), 



Sdb-Classes op Mammals. 



1. With long toothless jaws like a ducl^'s 



billjayingeggs in a mammary pouch. Ornitliodelphin: Duclibill. 



2. With a poucli for holding the yonvg . Marsupialia: Opossum. 



3. With a placenta; brain in most cases 



with convolutions. Monoddpliia: Rat, Dog, etc. 



Sub-Class I. — Ornithodelphia. 



General Characters of Monotremes.— The duckbill and 

 spiny ant-eater (Fig. 284, Eclddna liystrix) are tlie 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 eight horny teeth in the 

 duckbill), which are long and narrow in the Echidna, or 

 broad and flat in the duckbill {Ornithorliynclius ■para- 

 doxus), where it is covered by a leathery integument; the 

 external ear is wanting. Both animals lay true eggs. 



In the aquatic duckbill (Fig. 282) the feet are webbed, 



