84 The American Geologist. February, i;)Oi. 
lusca, the Pluteus of the echinoderms, the Nauplius and Zoea 
of crustaceans. 
At any rate in looking over this phase of fossil forms in 
the Hall collection the skeletal development of the trilobites is 
an impressive fact. 
If we take the enumeration of species given by Walcott in 
Bulletin U. S. Geological Survey No, 30, we find the relative 
percentages of the zoological elements of the Cambrian faunas 
in North America to be as follows : 
Algae 2.30 Brachiopoda i7-04 
Spongiae 3.30 Lamellibranchs 25 
Hydrozoa 1.27 Gasteropoda 7-37 
Crinoidea 76 Pteropoda 5.09 
Annelida 1.27 Crustaceans 3.81 
Trilobites 5750 
The number of species in the trilobites more than equals, in 
its percentage of all ( ?) tabulated species, that of all the re- 
maining forms of life. And when we consider their numerical 
proportion in individuals, and examine the blocks of friable 
sandstone from St. Croix or Trempeleau, Wis., w^e find they 
rival in individual enumeration the brachiopods. In short the 
trilobites in the Cambrian clay clearly suggest biological prefer- 
vtent. 
In the ninth lecture in Dr. Brooks' series on the Founda- 
tions of Zoology wherein the question discussed is natural se- 
lection and the antiquity of life, the author places, after the ini- 
tial stages of zoological development, the scene of the faunal 
growth on the floor of the ocean. He says, "I shall give reasons 
for seeing, in the Lower Cambrian, another period of rapid 
change, when a new factor — the discovery of the bottom of the 
ocean — began to act in the modification of species, and I shall 
try to show that, while animal life was abundant long before, 
the evolution of animals likely to be preserved as fossils took 
place with comparative rapidity, and that the zoological feat- 
ures of the Lower Cambrian are of such a character as to in- 
dicate that it is a decided and unmistakable approximation to 
the primitive fauna of the bottom, beyond which life was rep- 
resented only by minute and simple surface animals not likely 
to be preserved as fossils." Dr. Brooks' assertion that "on the 
old Cambrian shore the same opportunity to study the embry- 
