MAN FROM THE FARTHEST PAST | 
prominently during the Silurian period which followed. 
Some of these chambered shells were of straight, tapering 
form, and no less than twelve to fifteen feet in length and 
one foot in maximum diameter. It is interesting to 
speculate on their means of locomotion and nutrition. 
The great period called the Silurian, which followed the 
Ordovician, is the last one dominated by invertebrate 
animal life. During the Silurian the trilobites declined, 
but the cephalopods continued very notable. They 
shared their prominence, however, with certain remark- 
able crustaceans, of somewhat scorpionlike appearance, 
called Eurypterus and Pterygotus. Among these latter, 
giants of one and a half to six feet in length appeared. 
These creatures have never been surpassed among crusta- 
ceans of all ages. Many of the other orders and classes 
of invertebrates flourished notably during the Silurian, 
as they had done previously, but towards the end of the 
age the third great life era, that of the vertebrates repre- 
sented by fishes, began to dawn. 
Here we find forms called the ostracoderms, which seem 
to constitute a connecting link between the crustaceans, as 
represented by the trilobites, and the true vertebrate 
fishes that were to come. Their heads and trunks ex- 
ternally bear resemblance to the trilobites, while present- 
ing fishlike fins and tails. They opened their jaws later- 
ally like the crabs, not vertically like the fishes, yet so fish- 
like were their bodies and tails that till recently students 
have always classed them with the fishes. 
With the Devonian period comes the reign of the true 
fishes, though the forms differed extensively from most of 
those in the present seas. Sharks lived in the Devonian 
both in the open sea and in brackish waters of the shores. 
Lampreys, lungfishes, and ganoids were also developed. 
But as yet paleontologists have found no trace of the now 
dominant bony fishes. 
Our fourth era of special interest is the heroic age of 
vegetation, the Carboniferous. As vegetation is indis- 
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