THE AMERICAN NATURALIST. [Vol. XXXVII. 



in Bothriolepis and Pterichthys, and by the relatively small size 

 of dorsal and caudal fins in the latter genus. Such a condition 

 is in marked contrast to that in many of the most primitive of 

 the true fishes where the pectorals are much smaller rela- 

 tively, serving rather as balancers, the principal work of locomo- 

 tion being performed by the tail and caudal fin. 



Among true vertebrates, the only structures suggestive of 

 the cephalic appendages of the Ostracoderms are the external 

 gills during their early embryonic stages, including among these 

 structures the "balancers" of Amphibian larva:. The large 

 size and anterior position of the latter appendages make them 

 especially suggestive of the oar-like appendages of Tremataspis. 



Hanover, N. H., 

 April 19. 



