EVOLUTION OF EARLY PALEOZOIC FAUNAS 195 
fauna of the Cambrian 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- 
edly present, and, together with the minute Protozoa, must have formed 
the primary food-supply.' 
The réle assigned by Dr. W. K. Brooks to microscopic forms was 
an important factor in Cambrian time, for the organisms found in the 
rocks of that period were mainly carnivorous, and were adapted either 
to straining minute organisms from the water, or to gathering them 
up from the bottom. 
Uniform marine physical conditions over the submerged portions 
of the continental platform in Lower Cambrian time are indicated by 
the uniformity of the fauna on opposite sides of the present continent. 
Whether this fauna was distributed between the east and the west to 
the north of the central land-area, or south of it, is not definitely deter- 
mined, yet the absence of Lower Cambrian rocks and fossils from the 
collections made in the Arctic region, and the presence of closely allied 
species in the Lower Cambrian rocks of Alabama and California, point 
to the southern coast-line as the probable highway for the distribution 
of the littoral fauna. Nothing that suggests the Lower Cambrian fauna 
is known from South America; in this case, deep water may have been 
the barrier. 
With the advent of Middle Cambrian time land-areas came into 
existence on the northeast, forming barriers which so affected marine 
conditions in relation to life that the Paradoxides fauna developed in 
the Atlantic basin and the Olenoides fauna in the Appalachian region 
south of the Champlain Valley. To the south and on all sides of the 
t W. K. Brooks, Studies from the Biological Laboratory, Johns Hopkins University, 
Vol. V, 1893, pp. 136-38. On p. 137 Dr. Brooks says: “‘The simplicity and abun- 
dance of the microscopic forms and their importance in the economy of nature show 
that the organic world has gradually shaped itself around and has been controlled by 
them.” 
