NO. I ABRUPT APPEARANCE OF THE CAMBRIAN FAUNA 1 3 



duced by a sinking of the ocean bed that lowered the shore hne of all 

 the continents. It was of world-wide extent and of great duration, 

 and it wasduring this period that the open-sea fauna, as described by 

 Brooks, probably found its way to the littoral zone and developed 

 in the protected waters along the ancient continental shelves. Of 

 this period we have no known record either in marine sedimentation 

 or in life. 



EMDENCE OF THE FOSSILS 



The evidence afiforded by the few traces of pre-Cambrian fossils 

 is inconclusive as far as determining whether their hal^itat was in 

 marine, brackish, or fresh water. 



The fossils from the Chuar group of Arizona [Walcott, 1899, P^- 

 23, figs. 1-4; pi. 27, figs. 9-13] are not sufficiently characterized to 

 prove their origin or habitat. The Protozoan, Cryptozoon ? occi- 

 dental c Dawson, is very abundant in Arizona, also in the Belt series 

 of Montana. Alberta, and British Columbia. It occurs in limestones 

 similar to those deposited in the fresh-water lagoons of Florida, and 

 similar to the limestones of the lake deposits of the Tertiary forma- 

 tions of the Great Plains region of North America. The fossils of 

 the Beltina zone of Montana and Alberta could as readily have been 

 developed in fresh or brackish waters. There is nothing about the 

 crustacean remains incompatible with their living in fresh water, 

 in fact, the fragments indicate a form more nearly related to the 

 fresh-water Branchiopoda with very thin test, rather than the strong 

 Mcrostome {Enryptcriis, etc.). 



The oldest Cambrian fauna now known, with Ncvadia zvccksi and 

 Hohnia ron'ci [Walcott, 1910, pp. 257 and 292], is limited to a few 

 forms, but with a careful examination of the region where it occurs in 

 southwestern Nevada it is highly probable that a considerable fauna 

 will be found. The strata in which it occurs were deposited in a 

 depression opening out toward the Pacific ocean, where southwestern 

 California is now located ; this depression soon extended northward 

 and presumably connected through to British Columbia and Alberta, 

 as the same species of OlencUus occur in the central and upper por- 

 tions of the Lower Cambrian both in Nevada and Alberta. 



I do not know of a Cambrian fauna as old as that of Ncvadia 

 wccksi on the eastern side of the continent, or on the European 

 continent. It appears to be a portion of the older fauna that is miss- 

 ing everywhere except in southwestern Nevada. I think it was 

 brought in bv tlie advancing Lower Cambrian sea from a sea to 



