EVOIA'TIOX OF Till-: III' MAX FACE 



381 



jaws (il the sliarks. which ai'f (■(|iii\a- 

 Iciit to the cart ila.L;'iiii>ii> cdi'c of the 

 upper ami lower jaws of all the \'erle- 

 hrales al)o\"c the sliark>. 



'I'he iislies called "gaiioiiU."" of many 

 diU'eivnt varieties, show a >hiiiy sui'- 

 face armature coveriii.i:- tlie lace, as also 

 the hodv. recalling the insects with 

 their hard outer shell. But the note- 

 worthy thing about tliis ganoid sort o[' 

 face is that the hard t'overiiig of the 

 face and jaws has a bony siihstrat uiu 

 which completely invests the primaiw. 

 underlying brain case and the primary 

 or gill-arch jaws. This bony skin even 

 extends inward along the roof of the 

 mouth, forming the primitive hard pal- 

 ate, and along the inner as well as the 

 outer side of the primary lower jaw. 

 forming the sheathing bones of the jaw. 

 which are typical of fishes and higher 

 vertebrates. In this early stage of ver- 

 tebrate evolution this bony mask lies 

 fully on the inner and outer surfaces, 

 hut in the later evolutionary stages of 



all classes of M'i'tehrates, these sheath- 

 ing l»one> gi'adually .-ink lielow the -ui-- 

 face, in j)i'opoi't ion as a new layei' of 

 skin is genei-ateil on Ihcir sui'faces. and 

 as this new surface layer hecomes 

 thicker the oi'iginal sheathing bones 

 linallv come to be buried deeply under 

 the skin and often tightly appressed to 

 the still deeper prinuiry Ijrain case and 

 primary jaws. 



There have come down to us from 

 the Age of lieptiles a great many fossil 

 reptiles and amphibians which show 

 this shell of bone still on the surface, or 

 very near tlie surface. Even in the 

 modern alligators and crocodiles and 

 turtles the bony mask lies immediately 

 below the tough skin. 



An aggressive looking amphibian 

 (Cacops) from the Permian of Texas 

 has the general type of face which was 

 destined to give rise by diverse modifi- 

 cations to the characteristic faces of 

 reptiles, and indirectly to those of the 

 higher types. It still has the mask of 



£ 



iirx 



All mammals have facial muscles, producing a more or less mobile face. Tlicsc imisclcs are very highly 

 developed in man. In a comparison of gorilla (young) and white man (adult), homologous groups are 

 seen, such as the muscles surrounding the eyes, the nasal muscles, and the muscles that lift the lip. One of 

 the greatest gaps in the evidence of the evolution of the face is that there has been found no intermediate 

 type between the immobile nonmuscular face of reptiles and the mobile muscular face of mammals. Com- 

 parative anatomy, however, shows how one may have e\olved into the other. Figure after Ru(je 



