22 ANIMALS OF THE PAST 



the forerunners of backboned animals. The 

 earhest of these consist of small bony plates, 

 and traces of a cartilaginous backbone from 

 the Lower Silurian of Colorado, believed to 

 represent relatives of Chimiera and species re- 

 lated to those better-known forms Holopty- 

 chius and Osteolepis, which occur in higher 

 strata. There are certainly indications of ver- 

 tebrate 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. Pe- 

 tersburg, and are thouglit to be the teeth of 

 some animal like the lamprey. 



A little higlier up in the rocks, though not 

 in the scale of life, in the I^ower Old Red Sand- 

 stone of England, are found more numerous 

 and better preserved specimens of another lit- 

 tle fish-like creature, rarely if ever exceeding 

 two inches in length, and also related (proba- 

 bly) 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 es- 

 sential for their preservation that they should 



