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 amphibians but only two fully developed pairs (neurocentra and pleurocentra) in the higher mammal-like reptiles and mammals. 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, carried 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:
| 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 primiates (including man). . . . . . . . | 2 | 3 | 3 | 3 | 3 |
Similarly in the hind foot the primitive reptilian phalangeal formula was:ⅠⅡⅢⅣⅤ23454; but the formula in the higher mammal-like reptiles and mammals was: ⅠⅡⅢⅣⅤ23333
[ 283 ]
