THE EARLIEST KNOWN VERTEBRATES 13 



Europe the sum of the measurable strata is much 

 greater than in North America. 



The earliest traces of animal life are found deeper still, 

 beneath something like eighteen to twenty-five miles of 

 rock, while below this level are the strata in which 

 dwelt the earliest living things, organisms so small and 

 simple that no trace of their existence has been left, 

 and we infer that they were there because any given 

 group starts in a modest way with small and simple 

 individuals. 



At the bottom, then, of twenty miles of rocks the 

 seeker for the progenitor of the great family of back- 

 boned animals finds the scant remains of fish-like 

 animals that the cautious naturalist, who is much given 

 to "hedging," terms, not vertebrates, but preverte- 

 brates or the forerunners of backboned animals. The 

 earliest of these consist of small bony plates, and traces 

 of a cartilaginous backbone from the Lower Silurian of 

 Colorado, believed to represent relatives of Chimsera 

 and species related to those better-known forms 

 Holoptychius and Osteolepis, which occur in higher 

 strata. There are certainly indications of vertebrate 

 life, but the remains are so imperfect that little more 

 can be said regarding them, and this is also true of the 

 small conical teeth which occur in the Lower Silurian of 

 St. Petersburg, and are thought to be the teeth of some 

 animal like the lamprey. 



A little higher up in the rocks, though not in the scale 

 of life, in the Lower Old Red Sandstone of England, are 

 found more numerous and better preserved specimens of 

 another little fish-like creature, rarely if ever exceeding 

 two inches in length, and also related (probably) to the 

 hag-fishes and lampreys that live to-day. 



These early vertebrates are not only small, but they 

 were cartilaginous, so that it was essential for their 



