28 ANIMALS OF THE PAST 



ated with true jaws, for the two do not, as might 

 be supposed, necessarily go together. Neither 

 did these animals possess hard backbones, and, 

 while Pterichthys and its relatives had arms or 

 fins, the hard parts of these were not on the 

 inside but on the outside, so that the limb was 

 more like the leg of a crab than the fin of a 

 fish; and this is among the reasons why some 

 naturalists have been led to conclude that ver- 

 tebrates may have developed from crustaceans. 

 Pteraspis, anotlier of these little armored pre- 

 vertebrates, had a less complicated covering, 

 and looked very much like a small fish with its 

 fore parts caught in an elongate clam-shell. 



The fishes that we have so far been consider- 

 ing — orphans of the past they might be termed, 

 as they have no living relatives — were little fel- 

 lows ; but their immediate successors, preserved 

 in the Devonian strata, particularly of North 

 Ainerica, were the giants of those days, termed, 

 from their size and presumably fierce appear- 

 ance, Titantichthys and Dinichthys, and are re- 

 lated to a fish, Ce?^atodus, still living in Australia. 



We know practically nothing of the exter- 

 nal appearance of these fishes, great and fierce 



