-2 
FORMATIONS OF ORDOVICIAN AGE. 3 
recognized in the Jacksonburg section, the species occurring in each 
bed being recorded in the column especially reserved for that bed. 
An examination of the table shows that the first sixteen beds, com- 
prising a thickness of strata of fifty-eight feet and four inches, con- 
tain a fauna which is quite distinct from the faunas of the succeeding 
strata. The most characteristic species of this lower fauna are Dal- 
manella subequata and Leperditia fabulites, and the beds containing 
it may be considered as representing the Black River limestone of the 
New York section. The beds from numbers 17 to 61, inclusive, repre- 
sent the Trenton limestone proper, and although there are several 
subordinate faunal zones recognizable, the beds, as a whole, are es- 
pecially characterized throughout by Plectambonites sericeus, Dal- 
manella testudinaria, Zygospira recurvirostris and Pterygometopus 
callicephalus. Only eight species pass from the lower horizon to the 
upper, and of these only three, Iafinesquina alternata, Isotelus gigas 
and Bumastus trentonensis, are conspicuous forms in both the lower 
and the upper faunas. The remaining five species are either com- 
paratively rare, or, if normally members of the lower fauna, as Dal- 
manella subequata, occur, also, only in the base of the beds carrying 
the upper fauna; or they are initiated near the summit of the lower 
beds and become more abundant above. 
Within the Trenton limestone proper a basal zone, including beds 
numbers 17 to 19, inclusive, is especially characterized by the species 
Streptelasma corniculum, Actinostroma trentonensis, Orthis tricenaria, 
Dinorthis pectinella and Rhynchotrema inequvalvis. Just above this 
basal zone is another, with rather indefinite limits, which bears Paras- 
trophia heniplicata. The lower beds of the Trenton limestone proper 
are characterized by the special abundance of Plectambonites seri- 
ceus and the rarity of Dalmanella testudinaria, but higher up the 
relative abundance of these two species is reversed, Dalmanella usually 
being the most conspicuous species in the faunas. In the higher beds 
the Pelecypod, Gastropod and Trilobite species become more con- 
spicuous. In a general way the faunas of the section may be sepa- 
rated into three zones—the Black River horizon, the lower Trenton 
and the upper Trenton—the two lower zones being more sharply 
separated than the two upper ones. 
The more or less isolated Trenton faunas which have been collected 
from other localities in the State can all be placed, in a general way, 
in one or the other of these three zones recognized in the Jackson- 
