FKEFACE. 



which, from being overlooked, have often led to the confounding of species 

 from difl'erent 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 palaeozoic 

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

 Tarieties 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 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 Leptana tenuistriata, and the 

 Calymtne senaria ; the first of which is regarded by some geologists as identical 

 with L. rugosa, and the latter with C blumenbachti. 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 

 extmction 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. 

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

 fossils. -The greater number of species previously known were described by 

 Mr. Conrad, in his Annual Reports on the Palaeontology of the State, from 

 1838 to 184 1 ; and in the Journal of the Academy of Natural Sciences of 

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

 ;n manuscript by Mr. Conrad, 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. 



