EVOLUTION OF EARLY PALEOZOIC FAUNAS 29 



of pre-Cambrian land, flanked on either side by large barrier 

 islands that served to protect straits, sounds, or seas from the open 

 ocean. Ocean currents flowed through the sounds with varying force 

 and volume, not only from the cold arctic waters to the north, but 

 from the warm tropical region to the south. The relative position of 

 land and sea is based on the present interpretation of the observed 

 characters and distribution of the pre-Cambrian and Lower Cambrian 

 rocks. The distribution of Lower Cambrian faunas indicates the prob- 

 able courses of the marine currents. A fundamental assumption is 

 that the great ocean basins and continental masses occupied their 

 present relative positions during at least the Algonkian portion of 

 pre-Cambrian time. 



The map of the continent at the close of Cambrian time shows that 

 during this period upon the continental area marked changes in the 

 positions of the land and sea took place. Broad shallow seas followed 

 the transgressing shore-line of Middle Cambrian time, offering most 

 favorable conditions for the long-continued development and distribu- 

 tion of marine life.' There were undoubtedly deep and shallow seas 

 and bays, cold and warm waters, strong and weak ocean currents of 

 unlike temperatures, protected bays with sandy and muddy bottoms, 

 shore lines gently sloping to deep water, and many conditions promot- 

 ing the evolution of the faunas through favorable or unfavorable 

 changes in environment, temperature, and food-supply. 



The sediments of Cambrian time are mainly those deposited near 

 the shore-line and in adjacent relatively shallow waters. There is 

 little if anything to indicate deposits of the abyssal sea. If the littoral 

 fauna of the Caml^rian sea had begun to work its way down the conti- 

 nental slopes beyond the continental margin into the depths, we can 

 find no evidence of it, either in the Cambrian rocks, or in the character 

 of the present deep-sea fauna. 



The life of Lower Cambrian time included Crustaceans (trilobites, 

 ostracods), Mollusca (gasteropods) , Molluscoidea (brachiopods) , 

 Vermes (annelids), Echinodermata (cystoids), Coelenterata (sponges, 

 corals, jelly-fishes), and the simplest animals, the Protozoa (rhizopods). 

 Immense quantities of microscopic, unicellular plants were undoubt- 



I See theoretic section at the close of Cambrian time: Bull. U. S. Geol. Survey, 

 No. 81, 1891, PI. II. 



