THE EARLIEST KNOWN VERTEBRATES 19 



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 be- 

 longing to types that have mostly perished, giving 

 place to others better adapted to the changed conditions 

 wrought by time. Almost the only living 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 scattered 

 through the rocks were arranged. For the teeth were 

 not planted in sockets, as they are in higher animals, 

 but simply rested on the jaws, from which they readily 

 became detached when decomposition set in after 

 death. To complicate matters, the teeth in different 

 parts of the jaws were often so unlike one another that 

 when found separately they would hardly be suspected 

 of having belonged to the same animal. Besides 

 teeth these fishes, for purposes of offence and defence, 

 were usually armed with spines, sometimes of consider- 

 able size and strength, and often elaborately grooved 

 and sculptured. As the soft parts perished the teeth 

 and spines were left to be scattered by waves and cur- 

 rents, a tooth here, another there, and a spine some- 

 where else; so it has often happened that, being found 

 separately, two or three quite different names have 

 been given to one and the same animal. Now and then 

 some specimen comes to light that escaped the thousand 

 and one accidents to which such things were exposed, 



