THE EARLIEST KNOWN VERTEBRATES 17 



From the better-preserved specimens that do now and 

 then turn up, we are able to obtain a very exact idea of 

 the construction of the bony cuirass by which Pterich- 

 thys and its American cousin were protected, and to 

 make a pretty accurate reconstruction of the entire 

 animal. These primitive fishes had mouths, for eating 

 is a necessity; but these mouths were not associated 

 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 vertebrates may have developed 

 from crustaceans. Pteraspsis, 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 considering 

 orphans of the past they might be termed, as they have 

 no living relatives were little fellows ; but their imme- 

 diate successors, preserved in the Devonian strata, par- 

 ticularly of North America, were the giants of those 

 days, termed, from their size and presumably fierce 

 appearance, Titanichthys and Dinichthys, and are re- 

 lated to a fish, Ceratodus, still living in Australia. 



We know practically nothing of the external appear- 

 ance of these fishes, great and fierce though they may 

 have been, with powerful jaws and armored heads, for 

 they had no bony skeleton as if they devoted their 

 energies to preying upon their neighbors rather than to 

 internal improvements. They attained a length of ten 

 to eighteen feet, with a gape, in the large species called 



