CLASS MAMMALIA. MAMMALS 



475 



Incisor 



Premolar 



Canine 



Jncisor 



Premolar 



CUPuIp 



Rrst motor 

 Fourth premolar 

 ■■ .' First molar 

 -r^ — Fourth premolar 



Teeth of adult dog f| , CJ, P j. M | x 2=42 



Pulp cavity 

 open at base 



Pulp cavity 

 opening small — -u 



Developing Completely formed 



incisor (human) incisor (human) 



Crown 



Neck 



Root 



Cross section 

 human molar 



Cross section 

 horse molar 



Molar (human) 



Molar (horse) 



Figure 347. Mammalian teeth. Above, /e/f, teeth of dog. The teeth of a mammal are of a 

 definite number and are collectively referred to as the dentition. Their number is expressed as 

 a dental formula that includes those of the upper and lower jaws of one side. The dental 

 formula of the dog is given and multiplied by two to show the total number of teeth. Upper 

 and lower, right, the structure of mammalian teeth shown diagrammatically in cross and longi- 

 tudinal sections. 



and large molars are useful for grinding food. 

 Not only can the kind of life of extinct ani- 

 mals be determined from their teeth, but 

 the type of contemporary plant life can be 

 reconstructed, and some conclusions can be 

 drawn regarding the no-longer-existing diges- 

 tive organs of these animals. 



Foot posture of mammals 



The primitive type of foot posture is 

 called plantigrade. The entire palm or sole 

 rests on the ground, and neither the wrist 

 nor the ankle is raised above the ground 

 (Fig. 348). This type gave way to the digiti- 



grade posture, the animals walking upon 

 their digits with the bones of the wrist and 

 ankle, and the upper ends of the palms and 

 soles raised above the ground. The third 

 type of foot posture is the unguligrade, 

 characteristic of hoofed animals. These un- 

 gulates walk on modified nails or hoofs, 

 such as those of the horse. 



Development of mammals 



The eggs of most mammals develop 

 within the body of the mother; the excep- 

 tions are the monotremes which lay eggs, 

 thus showing their close relationship to rep- 



