MAMMALIA 



287 



single pair connects with the dorsal aorta and the heart is 

 always four-chambered. 



The teeth of mammals show great variations which in 

 general are correlated with differences in feeding. The 

 reptilian ancestors of the mammals possessed teeth and all 

 existing mammals have them at some time. The curious 

 duck-bill, certain whales, armadillos and the ant eaters 

 have very rudimentary teeth, however, which do not break 

 through the jaw and are resorbed before birth. Most 

 mammals have two sets of teeth a temporary or "milk" 

 dentition and a permanent dentition. In some (guinea 



FIG. 105. A comparison of the heart (dotted) and chief arteries in: A, fish; 

 B, frog; C, lizard; D, bird; E, mammal. 



pigs, bats) the former is lost before birth. The teeth may 

 be all alike (homodont dentition), as in the dolphins, or 

 show a differentiation into incisors, canines, premolars, and 

 molars (heterodont dentition). The molars appear only 

 in the permanent dentition. Very often animals which 

 have a specialized heterodont dentition may lack certain 

 types of teeth, and a diastema, or bare space, is left in the 

 jaw. A squirrel, for example, has very large incisors but 

 no canines .and there is a gap in front of the premolars; a 

 cow lacks incisors in the upper jaw and has no canines 

 whatever. 



