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HISTORICAL GEOLOGY 



it is seldom, even in the sediments that are being deposited to-day, 

 that plants are embedded in marine sediments. 



Small calcareous algae have been found in the Cambrian rock of 

 the Antarctic Continent and elsewhere, and it appears probable that 

 plants which secreted lime played an important part in the formation 

 of the Cambrian limestone. 1 



Animals 



The oldest known fossiliferous rocks of the Cambrian contain a 

 varied fauna. Corals, sponges, worms (in the form of trails and 

 borings and impressions), brachiopods, pteropods, and crustaceans 

 have been identified, and there is no doubt that the discoveries have 

 brought to light only a fraction of the life of the time. Doubtless the 

 lowly protozoans were in existence at that time, as well as other classes 

 which have not been found. This diverse life, as has been shown, did 

 not arise suddenly, but was derived from a long line of ancestors which 

 lived during Proterozoic times. 



Crustacea 



Trilobites. — The highest and most striking form of life of the 

 period was the trilobites (Greek, tri-, three, and lobos, a lobe or rounded 



Fig. 379. Dorsal and ventral views of an Ordovician trilobite, showing 

 the restored appendages. (Beecher.) 



1 Garwood, E. J., — Nature, Vol. 92, 1913, p. 114. 



