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- 
