THE EARLIEST KNOWN VERTEBRATES 29 



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 in- 

 ternal improvements. They attained a length 

 of ten to eighteen feet, with a gape, in the large 

 species called Titanichthys, of four feet, and 

 such a fish might well be capable of devouring 

 anything known to have lived at that early 

 date. 



Succeeding these, in Carboniferous times, 

 came a host of shark-like creatures known 

 mainly from their teeth and spines, for their 

 skeletons were of cartilage, and belonging to 

 types that have mostly perished, giving place 

 to others better adapted to the changed condi- 

 tions wrought by time. Almost the only liv- 

 ing relative of these early fishes is a little shark, 

 known as the Port Jackson Shark, living in 

 Australian waters. Like the old sharks, this 

 one has a spine in front of his back fins, and, like 

 them, he fortunately has a mouthful of diversely 

 shaped teeth ; fortunately, because through their 

 aid we are enabled to form some idea of the 

 manner in which some of the teeth found scat- 



