PALEONTOLOGICAL GEOLOGY 
329 
Occurrence of Oldest Known Trilohites. The Cambric sections 
of western America have an extraordinary development. In the 
Silver Peak district, beyond Tonopah, the Early Carbric strata 
aggregate more than 6,000 feet. This Silver Peak section is es¬ 
pecially instructive because of the fact that it furnishes remains 
of the most ancient trilobites of which we have knowledge. Two 
forms in particular characterize this low horizon which is within 
200 feet of the base of the section exposed. 
The oldest trilobitic zone is designated the zone of the Nevadia 
weeksi fauna. The species represented are Nevadia weeksi, Wal¬ 
cott, and Holmia rowei, Walcott. These forms continue far below 
the Mesonacis gilberti fauna which belongs to the upper part of 
the Early Cambric formation. There are no other trilobites 
known to occur in the 5,000 feet of strata in which they are 
found except the Mesonacidean genera Nevadia, Holmia and 
Olenellus. Ptychoparia is represented by a single fragment some¬ 
where in the uppermost 400 feet of the section. 
The two species mentioned are all that have been discovered thus 
far in the lower beds. A fragment of perhaps an Olenellus occurs 
1000 feet above. The form known as Olenellus claytoni comes 
from an horizon 3000 feet above. 
fA noteworthy feature connected with this primitive fauna is 
that there are no traces of forms having a large pygidium. Early 
stages of growth of the young of all of the Mesconacidse repre¬ 
sented have a very large cephalon but a very small pygidium. Both 
of the trilobites described are characterized by minute pygidia. 
Nevadia weeksi has a large cephalon, 28 thoracic segments, and 
a very small plate-like pygidium, without a defined segment. 
Holmia rowei also has a large cephalon, 16 thoracic segments, 
and a very small pygidium which has one distinct segment. 
It is sometimes stated that the primitive trilobite was a flat, 
free-swimming form with subequal cephalon and pygidium. If 
this were so it would seem that forms with large pygidia should 
occur in great variety and numbers in the earliest Cambric 
faunas. These oldest forms do not sustain this contention. Micro¬ 
discus and Eodiscus occur at a much later stage in the Early 
Cambric sequence. 
Walcott. 
