OBSERVATIONS ON THE GENUS I'TERASPIS 521 



the large bony nuchal plate which supports the great dorsal spine, 

 and is constantly anchylosed to the posterior margin of the 

 skull. The rostrum of the Pteraspis is not without its analogue in 

 Loricaria. 



On the other hand, that rostrum might be compared with the 

 prolonged snout ol Acipenser; and the shield of Pteraspis presents 

 many points of similarity with the cephalic buckler of such fishes as 

 Dipterus, Diplopterus, and Osteolepis. 



