PUZZLES AND PATRIARCHS. 



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to the pearly layer of the oyster and other 

 similar shells; an outer very dense layer, in 

 structure resembling teeth, and a middle layer 

 of polygonal chambers. The dorsal spine seen 

 in the figure doubtless served the purpose of a 



FIG. 16. Three extinct ancestral forms of fishes. A. Pteraspis 

 rostrate, one of the Heterostraci. B. Cephalaspis lyelli. 

 C. Pterichthys, after Traquair. 



dorsal fin, as a balancing organ. The tail was 

 covered with a closely-fitting armature of scales. 

 About the structure of the fin we know very 

 little. The typical representatives of this order 

 belong to the genus Pteraspis, of which one species, 

 Pteraspis rostrata, is figured here (fig. 16 A). 



The Osteostraci are practically confined to the 

 Upper Silurian and Lower Devonian rocks. The 

 characteristic genus is Cephalaspis, the finest 



