AND THEIR DESCENDANTS 



in the upper Ordovician rocks of Sweden, and all we 

 can say of it is that it belonged to the same class of 

 insects as lady-birds. The existence of this insect shows 

 that there must have been land vegetation and an atmo- 

 sphere which was suited to active air-breathing things. 

 The other appearance of great interest in the Ordovician 

 rocks is that of the first fish. They were found in 

 Colorado, but they are very much shattered and tell us 

 very little about the animals they represent. These fish 

 were covered with plates, and were evidently thus de- 

 fended against attack, so that we may surmise the ex- 

 istence of some other animal that preyed on fishes. 

 Whether these fishes were themselves ferocious we cannot, 

 however, say. But that which was the chief characteristic 

 of the Ordovician era was the climax of the trilobite. 

 More than half of all the known trilobites were present in 

 Ordovician times. Only a few of these came over from 

 the Cambrian, while the others make their first appear- 

 ance in this period. In the next period (Silurian) their 

 numbers fell to one half, and in later periods declined 

 still further, till they disappeared altogether at the close 

 of the Palaeozoic era. Some of these curious animals 

 appear to have been able to move very quickly ; others 

 would roll themselves up like hedgehogs to defend them- 

 selves against attack ; and some of the larger ones were 

 from eighteen inches to two feet in length. Next to 

 these in interest were the cephalopod types, marine 

 animals, that may have resembled the swimming nautilus 

 of to-day in some of their developments. They attained to 

 enormous sizes, some of the shells being twelve to fifteen 



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