DEVONIC FISHES OF THE NEW YORK FORMATIONS 25 



the Heterostraci is there a noteworthy absence of bone lacunae and tubules 

 in the microscopic structure of the dermal plates, but recent discoveries of 

 entire bodies of Thelodus, Lanarkia, Drepanaspis etc. [text fig. i, 2] seem 

 to demonstrate that the hard shield of Pteraspidians arose through fusion 

 of separate shagreen tubercles and spinelets. Incontc stabh , therefore, the 



the dorsal fin as yet. (After Traquair) 



Agnatha must be looked upon as chordate animals ; yet it does not follow 

 that they are true fishes. On the contrary, the characters which separate 

 them from the latter class are most trenchant, and are not adequately 

 expressed in the taxonomic system except by allotting to them the rank of 

 an independent class. For instance, in marked distinction from fishes 

 proper, none of the Ostracophores exhibits the slightest trace of ordinary 



