THE LINEAGE OF MAN 



to man himself. The bony bar below this opening, variously 

 called the jugal or malar or zygomatic bone, may similarly 

 be traced upward to man. 



We likewise owe to these lower predatory reptiles the 

 reduction in number of the wedge-like pieces that finally 

 grew together to make a single vertebra or functional unit 

 of the backbone, there being four pairs in the early amphib- 

 ians but only two fully developed pairs (neurocentra and 

 pleurocentra) in the higher mammal-like reptiles and mam- 

 mals. These aggressive and progressive animals also took 

 many other steps toward the mammalian type of skeleton, 

 including the human type, most of them correlated with 

 their improved running powers ; for whereas the earlier forms 

 had crawled almost on their bellies with sprawling arms 

 and legs, the cynodonts, or later mammal-like reptiles, car- 

 ried their bodies well off the ground, almost like the more 

 primitive mammals, as we know from the detailed form of 

 their limb bones. 



Among the improvements introduced by the cynodonts 

 was the reduction of the phalangeaL formula (that is, the 

 number of bony segments or phalanges in the different 

 fingers and toes of each forefoot) to the mammalian number, 

 as indicated in the following table: 



NUMBER OF PHALANGES IN EACH DIGIT 



Digit I II III IV V 



In earlier reptiles 2 3 4 5 3 



In cynodonts > 2 3 3 3 3 



In primitive mammals 2 3 3 3 3 



In primates (including man) 2 3 3 3 3 



Similarly in the hind foot the primitive reptilian phalangeal 



I II III IV V 



formula was;— — - — ^ — - — t~ ' ; but the formula in the higher 



I II III IV V 

 mammal-like reptiles and mammals was: ~ ; ~ — ; — ~~ 



■•• 2 3 3 3 3 



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