OUR BEST FEATURES 



these plates are of exactly the same nature as the 



roof -bones of the skull and the scales on the body. 



Thus arises the hard "facial mask" so often 



referred to in the preceding pages. 



As used in this book the term "secondary jaws" 



is limited to the tooth-bearing plates covering the 



external borders of the primary upper and lower 



jaws. There are three of these elements on each 



side of the head throughout the series (Fig. 50) 



from fish to man and their amazing constancy is an 



item of evidence of the unity of plan and origin of 



all the higher vertebrates. The first of these 



secondary jaw elements is the premaxilla, one on 



each side of the mid-line, at the front end of the 



jaw; this is followed by the maxilla, one on each 



side behind the premaxillse. When we compare 



the under side of the skull (Fig. 63, I, II) of one 



of the fossil lobe-finned (crossopt) ganoids of the 



Devonian with that of one of the early amphibians 



of the Coal Measures, we can hardly doubt that 



the premaxilla and maxilla of the former are each 



completely homologous with the corresponding 



element in the latter. And from the earliest 



amphibian to man they can be traced in convincing 



detail (Figs. 50, 53). 



107 



