Tlie Era of Invertebrates 



coui'se of time were formed above the strata 

 containing the ^vorm burrows. 



The earliest incontestable record of life in 

 this country, if not anywhere, is in some rocks 

 called Algonkian, found in the Belt Mountains, 

 Montana, and Grand Canon region, in which 

 have been found a few shells. Like the earliest 

 remains of fishes, these are in an imperfect state 

 of preservation, although unmistakably the re- 

 mains of animals, and animals related to those 

 found in the rocks above. In these overling 

 rocks, constituting the Cambrian system, so 

 called because it was first studied extensively 

 in Wales, the Cambria of the Romans, evi- 

 dences of life are met with in comparative 

 abundance. It is not so very many years ago 

 that a few species of shells represented the en- 

 tire known life of this great series of rocks, but 

 careful search has led to the discovery of numer- 

 ous species of brachiopods and trilobites, as weU 

 as a few mollusks, and now over one hundred 

 and seventy species of animals are known, from 

 the very lowest portion of the Cambrian, and 

 over ^Y^ hundred species from the entire system. 



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