22 ANIMALS OF THE PAST 
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 Chimera 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 thought to be the teeth of © 
some animal like the lamprey. 
A little higher up in the rocks, though not 
in the seale of life, in the Lower 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 
