OUR BEST FEATURES 



to the increase in size of the pterygoid muscles 

 may safely be ascribed the pinching together of 

 the opposite pterygoid bones and the development 

 of a high bony crest on the mid-line of the base 

 of the braincase (Fig. 53 V). 



Turning again to the teeth, we may summarize 

 their early history as follows: In some of the 

 Silurian ostracoderms (Lanarkia) the teeth of 

 later vertebrates are represented by thorny sha- 

 green denticles embedded in the skin all over the 

 surface of the body, but the ostracoderms them- 

 selves did not have teeth in the mouth. In the 

 sharks the skin on the inside of the mouth and 

 jaws carries the teeth, which represent only 

 enlarged dermal denticles. In the sharks the 

 tooth-bearing skin on the inner side of the jaws 

 is rolled inward in a spiral manner and as the old 

 teeth are broken off the new ones unwind or rotate 

 into place. 



In the lobe-finned or crossopt fishes, representing 

 the ancestors of the amphibians, at least the larger 

 teeth arise from pockets of bone sunk below the 

 surface of the bony enamel-covered skin. In these 

 forms the bases of the teeth are deeply and 

 complexly infolded and the pockets of bony skin 



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