14 



ANIMALS OF THE PAST 



preservation that they should 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 sur- 

 rounding 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 naturalists believe these little creatures to be re- 

 lated 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. 



Cephalaspis and Loricaria 

 An Ancient and a Modern Armored Fish. 



Still higher up we come upon the abundant remains of 

 numerous small fish-Uke animals, more or less com- 

 pletely clad in bony armor, indicating that they lived 



