MAMMALIA. 313 
So much difference of opinion obtains as to the best foynda- 
tion upon which to establish a division of the Mammalia into 
ereat primary sections, that it has been thought advisable to 
leave this subject wholly out of consideration. For our present 
purpose it is enough to adopt the old classification of Mammals 
into the two great divisions of the Placental and Non-placental 
forms. In the Placental Mammals the young is nourished with- 
in the body of the mother by means of a structure called the 
placenta, through which the nutrient materials of the mother’s 
blood reach the young. Inconsequence of this, the young of the 
Placental Mammals can be retained within the body for a con- 
siderable period, and when born, they are able to obtain their 
natural food—the milk—by their own exertions. In the Non- 
placental Mammals, on the other hand, the young are born at 
an extremely early period of their development, before there is 
any necessity that a Placenta should be formed for the nourish- 
ment of the foetus. In these cases, therefore, the young when 
born are much more immature and helpless than in the case 
of the Placental Mammals. So helpless are they, that they are 
even unable to suck, and have in most cases to be fixed by 
the mother herself upon the teats, whilst the milk is forced into 
their mouths by a muscle which is spread over the mammary 
gland. Adopting these primary sections as practically sufficient 
in an elementary work, the whole class of the Mammalia may 
be divided into the following fourteen orders :— 
Division A.—APLACENTAL MAMMALS, 
Order 1.—Monotremata. 
Order 2.—Marsupiala. 
Division B,—PLACENTAL MAMMALS. 
Order 3.—LZdentata. 
Order 4.—Szrenia. 
\ Order 5.—Cetacea. 
Order 6.— Ungulata. 
Order 7.—Hyracoidea. 
Order 8.—Froboscidea. 
Order 9.—Carnzvora. 
Order 10.—LRodentia, 
Order 11.—Cheiroptera. 
Order 12.—JLusectivora. 
Order 13.—Quadrumana. 
Order 14.—Bimana. 
