OUR BEST FEATURES 



squamosal bone covering the side of the temporal 

 part of the skull, by which time it had succeeded 

 in crowding its fellows quite off the map. 



Meanwhile, how did the crossopts and early 

 amphibians acquire the strong teeth with which 

 they carried on their predatory lives? In the 

 most primitive sharks (Fig. 5) the shagreen-bearing 

 skin is rolled around over the upper and lower jaw 

 cartilages and as the old teeth are broken off the 

 new teeth are gradually pushed up into place on 

 the edge of the jaws in a continuous succession. 

 In the typical sharks the tooth-bearing roll of 

 skin lies in a depression in the calcified cartilaginous 

 primary upper and lower jaws, but the teeth are 

 not separately connected with the jaws and when 

 in use are tied in place only by the strong dental 

 ligament attached to their bases. 



In the crossopts (lobe-finned fishes) of the 



Devonian period the primary upper jaw (palato- 



quadrate), now completely saturated with bone 



cells, is covered with bony dermal tracts bearing 



teeth, some very large and compressed, some 



small and conical. In front there is also a pair of 



tooth-bearing dermal plates, the pre vomers, on 



either side of the mid-line. Likewise the secondary 



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