394 SYSTEMATIC ZOOLOGY. 



21. The Mammalia have hairy skin, a four-chambered 

 heart, warm blood, and two occipital condyles. 



22. The young are nourished by milk furnished by the 

 mother. 



23. The teeth have roots, and usually several kinds of 

 teeth are recognizable. 



24. The Mammals are divided into Monotremata and 

 Eutheria. 



