314 MAMMALIA. 



CLASS I. MAMMALIA. (THE MAMMALS.) 



A Mammal is a warm-blooded, air-breathing vertebrate, having 

 the skin more or less hairy (or rarely naked) ; viviparous, the em- 

 bryo developed from a minute egg destitute of food-yolk (except 

 in the Monotremata, in which group the eggs are large, as in Rep- 

 tiles, and are developed outside the body) ; the young nourished for 

 a time after birth by milk, secreted in the mammary glands of the 

 mother ; respiration never by means of gills, but after birth by 

 lungs, suspended freely in the thoracic cavity, which is completely 

 separated from the abdominal cavity by a muscular septum (the 

 diaphragm) ; heart with four cavities ; a complete double circula- 

 tion ; blood warm. Skeleton more firm than in other Vertebrates, 

 the bones containing a larger proportion of salts of lime. Skull 

 articulating with the atlas by means of two occipital condyles; 

 bones of face immovably joined by sutures ; each half of lower jaw 

 of a single bone, articulating directly with the skull, the quadrate 

 bone becoming one of the bones of the ear (the malleus). Brain 

 case comparatively large, corresponding with the increased devel- 

 opment of the brain. The numerous other peculiarities of the 

 skeleton and the viscera need not be noticed in this connection. 



The following analysis of the Orders of Mammals which occur 

 within our limits is mostly taken from Professor Gill's " Arrange- 

 ment of the Families of Mammals.'* 



Orders of Mammalia. 



a. Young developed within the uterus from a minute egg which is destitute 



of food-yolk; milk glands with nipples ; no cloaca. (EUTHERIA.) 

 6. Young born when of very small size and incomplete development, never 

 connected by a placenta to the mother; brain small, its corpus callosum 

 rudimentary. (Subclass DIDELPHIA.) . . MARSUPIALIA, XLVII. 

 bb. Young not born until of considerable size and nearly perfect develop- 

 ment, deriving its nourishment, before birth, from the mother through 

 the intervention of a placenta ; a well developed corpus callosum. 

 (Subclass MONODELPHIA.) 



c. Brain with a relatively small cerebrum, which does not cover the other 

 ganglia, much of the cerebellum being exposed behind, and in front 

 much of the optic lobes. (Jneducabilia.) 

 d. Canine teeth none; incisors 3, rarely f, chisel-shaped; limbs adapted 



for walking GLIKES, XLVIII. 



dd. Canine teeth present, in some form ; incisors not | nor J. 



