PREFACE. 



Avhich, 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 Avork Avas 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 palreozoic 

 series furnishes its full proportion of fossils. The niuiiber of species and 

 A'arieties 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 palaeozoic series. 



Free from preconceived opinions regarding the geological range of species, 

 and Avilling 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 Leptana tenuistriata, and the 

 Cahjmme scnaria ; the first of which is regarded by some geologists as identical 

 Avith L. riigosa, and the latter Avith C. blumenbacliii. There are, hoAvever, some 

 slight differences in the external characters Avhich 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 

 extmction of species at the termination of any great epoch, form so small a 

 proportion of the Avholc, that they ofler very slender grounds for generaliza- 

 tion. 



The geological .structure and order of succession among the strata of this 

 period had ah-eady been clearly made known in tlie Reports of Messrs. 

 Vanuxem, Emmons and Mather, avIio have also given many of the typical 

 fossils. The greater number of species previously known Avere described by 

 Mr. Conrad, in his Annual Rcj)orts on the Pakeontology of the State, from 

 1838 to 1841 ; and in the Journal of the Academy of Natural Sciences of 

 Philadelpliia, Vol. viii, 1842. Several other species were named and described 

 ^n manuscript by Mr. Conrad, some of which Avere 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. 



