THE EARLIEST KNOWN VERTEBRATES 23 



be buried in soft mud as soon as possible after 

 death. Even if this took place they were later 

 on submitted to the pressure of some miles of 

 overlying rock until, in some cases, their re- 

 mains have been pressed out thinner than a 

 sheet of paper, and so thoroughly incorporated 

 into the surrounding stone that it is no easy 

 matter to trace their shadowy outlines. With 

 such drawbacks as these to contend with, it can 

 scarcely be wondered at that, while some natu- 

 ralists believe these little creatures to be related 

 to the lamprey, others consider that they belong 

 to a perfectly distinct group of animals, and 

 others still think it possible that they may be 

 the larval or early stages of larger and better- 

 developed forms. 



Still higher up we come upon the abundant 

 remains of numerous small fish-like animals, 

 more or less completely clad in bony armor, 

 indicating that they lived in troublous times 

 when there was Hterally a fight for existence 

 and only such as were well armed or well 

 protected could hope to survive. A parallel 

 case exists to-day in some of the rivers of South 

 America, where the little cat-fishes would pos- 



