Introduction 
between the ridges on the large teeth of the horse, cow, ané 
other similar animals. 
The number of teeth varies greatly in different animals and 
furnishes us with an excellent aid to classification. Sometimes 
teeth are entirely wanting, as in certain whales, and again we 
find one or other of the groups of teeth lacking, as the canines 
in the gnawing mammals, or the incisors in the upper jaw or 
the cattle and deer. 
In other families of mammals special names are used fo: 
some of the teeth; thus it will be noticed that in all carnivorous 
mammals one of the back teeth on each side of the jaw is muct. 
larger than the others, sometimes it is a molar, sometimes a 
premolar, but from its peculiar prominence it is called the car- 
nasal tooth. Again, in the insectivorous mammals, the incisors, 
canines and some of the premolars are all simple in structure 
and so much alike that they cannot be separated by their struc- 
ture; they are therefore for convenience known collectively as 
the unicuspid teeth. 
In many mammals some of the teeth become immensely 
developed and are termed tusks as, for example, in the elephant, 
walrus, narwhal, etc. 
Legs and Feet.—Next to the variations in their skulls and 
teeth mammals exhibit most diversity in the structure of their 
limbs. The limb of a mammal consists of four parts, and the 
bones which compose the fore limb have different names from 
those of the hind limb; thus we have 
FORE LIMB HIND LIMB 
]. Humerus (upper arm). Femur (thigh). 
I]. Ulna and radius (fore-arm). Tibia and fibula (lower leg). 
Ill. Bones of the carpus (wrist). Bones of the tarsus (ankle). 
IV. Phalanges (fingers). Phalanges (toes). 
The two bones composing the lower leg or calf which lie 
side by side are frequently joined together, or else the fibula is 
only partially developed. 
It is in the bones of the hands and feet, however, that we 
find the greatest variation, especially in the long bones that form 
the back of our hand (metacarpals) and the instep of our foot 
(metatarsals) and which support the fingers and toes. These 
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