374 ^^N INTRODUCTION TO THE STUDY OF FOSSILS 



one fang (canine) and seven premolar and molar teeth. 

 Molars are distinguished by having no milk teeth preceding 

 them. 



The ankle-joint is always between the bones of the lower leg 

 (tibia and fibula) and the ankle bones (tarsus), never between the 

 two rows of tarsal bones as in birds and reptiles. A muscular 

 partition, — the diaphragm (Fig. 142, di.), divides the body 

 cavity into an anterior portion, the thorax, containing the com- 

 pletely four-chambered heart and the lungs, and a posterior por- 

 tion, the abdomen, containing the digestive canal and the excre- 

 tory and reproductive organs. All mammals, except the most 

 primitive, the Monotremata, are viviparous; that is, the egg 

 develops into a form more or less like the adult before leaving 

 the body of the mother. The young, before birth, is nourished 

 by the blood of the mother (except in the Marsupialia) through 

 the placenta, a spongy membranous mass attached to the walls 

 of the uterus ; after birth, by milk, — the secretion of the 

 mammary glands. 



Most mammals, as the horse and deer, live upon the surface 

 of the ground ; some, as the squirrel and monkey, are arboreal 

 or tree dwellers ; rarely do they fly, as the bat ; a few, such as 

 the mole, are fossorial (burrowing) ; some, as the muskrat and 

 beaver, have taken to an aquatic life in fresh waters, others, 

 such as the seal and whale, to a marine life. 



For discussion of a typical mammal see the cat, p. 324. 



Derivation of name. — > Latin mamma, the breast. The 

 young are nourished for a time after birth by milk secreted by 

 the glands of the mother's breast. 



Causes of extinction of mammals. — Animals with rela- 

 tively larger brains being more alert, and adaptable to new 

 conditions (Fig. 162), will survive in competition with smaller- 

 brained forms ; they will also get more food and take better 

 care of the young. Other characters, besides defective brain, 

 which gradually lead to the extinction of their possessors are 

 inadaptive tooth and foot structure, excessive bulk, or extreme 



