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 literally 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- 
