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 

 rostrata, 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 



