OUR FACE FROM FISH TO MAN 



In the modern sharks there is a fold of skin at the 



back of the upper border of the mouth that seems 



to foreshadow the maxillary or upper jaw bones 



of higher fishes (Allis). Underneath this fold of 



skin at the corner of the mouth are two labial 



cartilages embedded in muscles which apparently 



serve to draw forward the corner of the mouth 



(Fig. 6). A similar fold of tooth-bearing skin 



(Figs. 50, 53) in the lobe-finned ganoids, or 



crossopts, gives rise to the premaxillary and 



maxillary bones, which have every appearance of 



being homologous with the bones that bear the 



same name in the earliest amphibians, and from 



thence these two bones can be followed through 



the mammal-like reptiles to the earliest mammals, 



thence through the ascending grades of primates 



to man. In the earlier crossopts these bones were 



covered with enamel and lay right on the surface 



but in the more advanced crossopts the ganoine 



layer has disappeared and the outer surface of 



the bone is rough, indicating that it was covered 



with a thick tough skin. The dentary bone of 



the lower jaw was likewise covered. 



In the early amphibians and reptiles the pre- 



maxilla, maxilla and dentary were likewise rough- 



130 



