occupy the same positions, and have the same functions. 
This the 
the positions of which may be seen in the accom- 
is also the case with internal organs, 
panying figure. 
The habits of Mammalia show that they possess 
a certain amount of intelligence, which may be arti- 
ficially stimulated by man, but which also increases 
with the age and experience of animals. Many 
possess constructive faculties, which they employ in 
Some ani- 
mals have a strong tendency towards migration, which 
is probably increased by scarcity of food. Other 
species hibernate, or sleep through the winter, during 
which time they take no food, their breathing be- 
comes very slow, and the temperature of the blood 
sinks nearly to freezing-point. 
making nests or burrows for themselves. 
There are no venomous species among mammals, 
but some are dangerous to man from their size and 
strength, while others are destructive to useful crops, 
as well as to the products of human industry. But 
many kinds are useful to man either alive or dead, 
and others are preserved as domestic animals. 
There are about 3000 species of Mammalia 
existing on the earth at present. Besides these, the 
petrified remains of many other species have been dis- 
covered, which throw great light on the former 
history of our earth, and on the present distribution 
and condition of the animal world. 
The organs of animals are all adapted to their 
manner of life. This is especially the case with the 
teeth, and with the organs of motion, which are con- 
sequently of the greatest importance in the classi- 
fication of Mammalia, 
The Teeth of Mammalia. 
The jaws of Mammalia usually contain a series 
of teeth in separate sockets. A perfectly developed 
tooth is divided into the crown, which is the exposed 
portion; the neck, or the part covered 
by the gum; and the roots or fangs, 4% 
which are fixed in the jaw. The 
hard portion of the tooth is composed a 
of a substance called dentine, or 
ivory, and this encloses a hollow 
called the pulp-cavity, which is filled 
up with 
£ 







i 
Section of Molar 
nerves and_ blood-vessels. Tenth eee Mak 
The crown is encompassed by a hard 
c. Cement d Den- 
substance called enamel, and the tine. e. Enamel 
: Fang. k Crown. 
fangs are surrounded by a bony crust, , Neck p. Pulp- 
known as the cement. cavity. 
According to their shape and position, teeth are 
divided into incisors (cutting-teeth, or front-teeth), 
canines (dog-teeth, or eye-teeth), and molars (back- 
teeth, double teeth, or grinding-teeth). 
The latter are of very different forms, according 
to the food of the animal, and we shall frequently 
have occasion to refer to them. 
The tecth are not always fully developed, accor 
ding to the description which we have given. Thus 


the enamel may be absent (as in the Sloths) or it 
may be absent on one side (as in the incisors of the 
Rodentia). Sometimes, too, the fangs are absent, 
and in this case, the tooth continues to grow during 
the whole life of the animal (as in the incisors of 
the Elephant, and of the Rodentia). On the other 
hand, the enamel may; 
penetrate into the den- 
tine, and form several 
layers, which become 
visible on the upper 
Teeth of the left upper jaw in man. 
surface of the tooth (as s Incisors. e. Canine Tooth. 
in the molars of the hb Molars (2 small and 3 large). 
Rodentia, horses and Ruminating animals). When 
these tolds completely cover the divisions of the den- 
tine, we have a compound tooth (Elephant; molars). 
The number of teeth is indicated by a special 
formula. a or more briefly, 
ae This means that man possesses on each side 
of each jaw 2 incisors, I canine and 5 molars. The 
first set of teeth, which fall out soon after birth, are 
called milk-teeth. They are only lost a short time, 
and are then replaced by permanent teeth. The 
hinder molars are not found in the set of milk-teeth, 
and therefore are never changed, when once developed. 
The full formula of the milk-teeth in man is as 
ey ee) - 
follows : SSS. 

In man this is 

Structure of the Feet in Mammalia. 
If we examine the hands and feet in the ske- 
leton of a man, we shall find that they are composed 
of a considerable number of bones. There are eight 
small bones of about equal size in the wrist, which 
are arranged in two rows. The ankle is composed 
of seven bones, which differ much in size, and the 
largest projects in such a manner as to form the heel. 
The middle of the hand and foot each contain 
five bones. These are jointed to the five fingers 
and toes, each of which is composed of three joints, 
except the thumb and the great toe, which have 
only two. The thumb can be opposed to the fingers, 

Bones of arm or fore feet of various Mammalia. 
r, Radius. u Ulna’ a.b.c.d.e.f. g h. Bones of Wrist 
1—5 Digits. I. Man. II. Dog. Ill. Pig. [V. Ox V. Tapir. VI Horse. 
and some animals have the power of placing the 
great toe in a similar position, though the foot is 
not thereby converted into a perfect hand. The 
ereat toe is partially opposible among certain races 
of savages, who sometimes row with their feet; and 
DSi 
