314 MAMMALIA. 
CiAass 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 eroup 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, ened 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 maileus). Brain 
case comparatively large, corresponding with the increased devel- 
opment of the brain. ‘The numerous other peculfarities 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 DipELpHiaA.) . . MArsuprauia, XLVII. 
6b. 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. (/neducabilia.) 
d. Canine teeth none; incisors 2, rarely $, chisel-shaped; limbs adapted 
for walking. . . ee a ew Bele Oe) ES ae 
dd. Canine teeth present, in some form; incisors not 3 nor 4. 
