42 FOURTH REPORT — 1834. 



suppose that the greater part of the tertiary tract will furnish 

 even a less proportion of living species than one fifth, vphile the 

 tertiary beds of North Carolina have contributed a group of shells 

 of which nearly two thirds are of recent species. The latter 

 territory would therefore most probably come within the plei- 

 ocene epoch, while the former districts are pretty clearly of the 

 American meiocene. It is an interesting fact, however, that our 

 meiocene shells, if we can at present call them such, resemble 

 most the species of the European older pleiocene. 



The following brief details embrace the results of a compari- 

 son of the respective fossils of each principal locality of our 

 middle strata, according to present data. 



New Jersey. — To begin with the locality in New Jersey, it 

 will be shown that we can at present enumerate only thirteen 

 species whose relations are established. Of these, twelve are 

 extinct, and one is supposed to be recent. 



What is curious in this small list is its containing so small a 

 proportion of species recent on our coast, though the deposit 

 evidently does not belong to our older tertiary, or eocene. The 

 species are either the same as those found in the meiocene or 

 middle tertiary of Maryland, or where they differ they are mostly 

 analogous. It is certainly not fair to reason from such very 

 limited data as are furnished us by this small list of fossils. New 

 additions to our present rather small catalogue of recent shells 

 may materially lessen the proportion of the species regarded as 

 extinct : anticipating this, I feel the less hesitation in separating 

 the beds of New Jersey from the eocene period. I consider it, 

 nevertheless, possible that some of the middle tertiary forma- 

 tions of this country may ultimately exhibit very nearly eocene 

 proportions, while the character of a majority of their fossils 

 may mark them to be decidedly meiocene in their relations. 



St. Mary's, Maryland. — This place has furnished about fifty- 

 six species, thirteen of which are recent on our coast, while the 

 remaining forty-three are extinct. The proportion of recent 

 species here is 23 per cent. 



Euston, Maryland. — The deposit upon the Choptank river, 

 near Easton, has presented, so far, about twenty-six species, 

 twenty-two of them extinct and four living. Among the extinct 

 species, the Perna inaxillata is conspicuous, as it always is 

 wherever the deposit shows a large preponderance of species no 

 longer living. The recent species here are about 16 per cent, 

 of the whole, placing the bed, like that of New Jersey, perhaps 

 in the meiocene period. 



Siiffolk, Virginia. — Here the total number of species procured 

 is forty-five, about thirty-five of which are extinct and ten re- 



