198 CHARLES D. WALCOTT 
Pteropoda.—The forms representing Orthotheca are abundant, 
large, strong, and evidently as well developed as those of the Middle 
Cambrian. 
Crustaceans.—The trilobites thus far found at this horizon are 
confined to two species of the genus Holmia. One of them, Holmia 
weeksi, new species, has many segments, and is more primitive than 
such forms as Olenellus thompsoni Hall’ and Holmia bréggeri (Wal- 
cott)? of the upper portions of the Lower Cambrian section. The other 
species, HTolmia rowei, new species, is of the same general type as 
Holmia bréggeri. The absence of all other trilobite genera is the most 
marked feature of this early Cambrian fauna. 
In the section 100 miles to the south, at Resting Springs, Inyo 
County, California, a brachiopod closely related to Billingsella high- 
landensis Walcott’ occurs 2,800 feet below the upper limestone, in 
association with the trilobite Holmia rowet. 
Comparing the species in the early Lower Cambrian fauna with the 
Olenellus fauna, in strata 5,000 feet higher in the section, we find a 
marked advance in the variety of the later fauna, but we do not know 
how much of this may be due to the absence, from our collections, of 
genera and species that may have existed during the deposition of the 
earlier sediments. In the earlier fauna of the Waucoba section the class 
characters of the Arthropoda, Mollusca, Molluscoidea, Vermes, and 
Coelenterata were developed, and while the study of the genera and 
species adds a little more to our knowledge of the rate of convergence 
backward in geologic time of the lines representing the evolution of 
animal life, it, at the same time, proves that a very long time-interval 
elapsed between the beginnings of life and the epoch represented by 
the Olenellus fauna. 
DISTRIBUTION OF THE LOWER CAMBRIAN (OLENELLUS) FAUNA OVER 
THE NORTH AMERICAN CONTINENTAL PLATFORM OF 
CAMBRIAN TIME 
The Olenellus fauna lived on the eastern and western sides of a 
continent that rudely outlined, in its general configuration, the North 
t See Bull. U. S. Geol. Survey, No. 30, 1886, p. 167. 
2 See Tenth Annual Report, U. S. Geol. Survey, 1891, p. 638. 
3 Proc. U. S. National Museum, Vol. XXVIII, 1905, p. 237- 
4 Tenth Annual Report, U. S. Geol. Survey, 1891, p. 595. 
