OUR ANCIENT RELATIVES 



of these bones by the jaw muscles, which finally 

 crept over and completely submerged the bone. 

 Thus by the time we reach the primitive mammal 

 stage of evolution almost the entire bony mask, 

 which had originated as the bony skin on the sur- 

 face, is now found covered by the facial and 

 jaw muscles. 



The relatives of the opossum and other primitive 

 pouched mammals until several years ago were 

 the only mammals of Cretaceous times of which 

 anything definite was known as to their skull 

 structure. In 1924 and 1925, however, Roy C. 

 Andrews and his colleagues of the American 

 Museum of Natural History discovered in the 

 Cretaceous formation of Mongolia half a dozen 

 imperfectly preserved skulls which appear to 

 represent the forerunners of the higher or placen- 

 tal mammals (see also Fig. 77 iv below). These 

 little skulls, which have been described by the 

 present writer with the collaboration of Dr. G. G. 

 Simpson, bring strong evidence for the conclusions 

 of Huxley, Henry Fairfield Osborn, Max Weber, 

 W. D. Matthew and others that the remote 

 ancestors of the placental or higher mammals of 



the Age of Mammals were small insectivorous 



51 



