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, another 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 
America, 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, Ceratodus, still living in Australia. 
We know practically nothing of the exter- 
nal appearance of these fishes, great and fierce 
