x PREFACE. 
which, from being overlooked, have often led to the confounding of species 
from different strata under the same name; and I have also endeavored to 
show how important, in some instances, are very slight differences. 
At the time this work was commenced, about seventy species were 
accurately known and described from all the strata of the lower division of 
the system. This number is already more than quintupled, and new forms 
frequently come under observation, showing that this part of the paleozoic 
series furnishes its full proportion of fossils. The number of species and 
varieties already described amounts to 381;* and among these we shall 
observe a proportion of the different classes and orders, not materially varying 
from other and younger portions of the paleeozoic series. 
Free from preconceived opinions regarding the geological range of species, 
and willing to find identical species in rocks widely separated, I have been 
surprised at the result of my investigations in the lower strata, which thus 
far have not produced a single species that can be satisfactorily established 
as common to succeeding formations. There are two species, concerning 
which some doubt may remain : these are the Leptena tenuistriata, and the 
Calymene senaria ; the first of which is regarded by some geologists as identical 
with LZ. rugosa, and the latter with C. blumenbachit. There are, however, some 
slight differences in the external characters which lead me to question the 
identity in either case, and to refer them to distinct species. These two in- 
stances, even if regarded as exceptions to the general rule of the entire 
extinction of species at the termination of any great epoch, form so small a 
proportion of the whole, that they offer very slender grounds for generaliza- 
tion. 
The geological structure and order of succession among the strata of this 
period had already been clearly made known in the Reports of Messrs. 
VanuxemM, Emmons and Martner, who have also given many of the typical 
fossils. The greater number of species previously known were described by 
Mr. Conran, in his Annual Reports on the Paleontology of the State, from 
1838 to 1841; and in the Journal of the Academy of Natural Sciences of 
Philadelphia, Vol. viii, 1842. Several other species were named and described 
in manuscript by Mr. Conran, some of which were published by Dr. Emmons 
in his Report upon the Second Geological District ; and I have been able to 
* See Table at the end of this volume, page 330. 
