Introduction 



between the ridges on the large teeth of the horse, cow, and 

 other similar animals. 



The number of teeth varies greatly in different animals ancj 

 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 carnivorouc 

 mammals one of the back teeth on each side of the jaw is much 

 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 



I. Humerus (upper arm). Femur (thigh). 



II. Ulna and radius (fore-arm). Tibia and fibula (lower leg). 



III. 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 



