213 PROCEEDINGS OF THE ACADEMY OP [1879. 



and sometimes exactly, certain geological horizons, and although 

 we cannot in most instances, as Prof. Huxley has forcibly pointed 

 out, positively, or even approximately, correlate, as far as age is 

 concerned, distantly separated formations, we can to a very great 

 extent correlate the cosmical conditions under which the forma- 

 tions in question were deposited. The correct determination, 

 therefore, of all organic remains is one of the greatest import- 

 ance. 



Unfortunately for the development of the science, the doctrine 

 that identical specific forms cannot, or ought not, occur over 

 widely separated areas has taken root in the minds of a few of the 

 most eminent scientific investigators, the baneful effects of whose 

 authority in relation to this special line of research, will be en- 

 countered by the student at almost every step in his investigations. 

 The science of geographical palaeontology, at least as far as the 

 invertebrata are concerned, ma}' be said to be in this country still 

 in a state of infancy, a circumstance parti}' due to the limited 

 number of workers in the field, and partly to the influences just 

 stated. 



I have endeavored in the following pages to summarize as nearly 

 as possible the results obtained from a series of comparisons be- 

 tween the Eocene mollusca of Western Europe and that of the 

 Southeastern United States, undertaken with the view of deter- 

 mining with a certain amount of precision the number of identi- 

 cal and very nearly related species. My comparisons were in a 

 number of cases made between actual specimens, and those mainly 

 determined in the localities to which they belong ; where no speci- 

 mens could be obtained I was compelled to content myself with 

 the various analyses of the descriptions and figures afforded by the 

 works of the most competent authorities. The result shows a far 

 greater intimate relationship to exist between the two faunae than 

 one is led to suppose from an examination of the "Check List" 

 prepared by Mr. Conrad for the Smithsonian Institution for 1866, 

 where, in fact, only about five species are enumerated as common 

 to the Eocene deposits of both shores of the Atlantic, viz. : Ostrea 

 ( Gryphostrea) eversa, Deshayes ; Cardita planicosta, Lam. ; ? Pho- 

 rus (Onustus) extensas, Sow.; HipjJocrenes columbaria, Defr. ; 

 and (?) Voluta (Volutililhes) mutata, Desh. In addition to these 

 Conrad enumerated in 1833 ("Fossil shells of the Tertiary Forma- 

 tions," p. 34) the following: Solarium patulum, Lam.; S. canali- 



