ARTICLE IX. 



Description of some new Fossil Shells, from the Tertiary of Petersburg, Virginia. By 

 Henry C. Lea, Philadelphia. Read May 29, 1843. 



IN the United States, the various Supercretaceous, or Tertiary, formations, are widely 

 extended and beautifully developed. They have been traced from the St. Lawrence to 

 the Gulf of Mexico, and from the Atlantic far into the western territory. For the sake 

 of convenience, geologists have divided them into two groups, according to their geogra- 

 phical position, namely, the Atlantic and the Western Tertiary. Of the latter it is not, 

 at present, my intention to speak, but little, comparatively, is known of it, and it 

 presents a rich field to any enterprising geologist who may undertake its investigation. 



The northern extremity of the Atlantic Tertiary, as far as it has been discovered, is 

 on the St. Lawrence and around Lake Champlain. From thence to Maryland, its 

 course may be traced by a few small patches in New Jersey, and, from Maryland to 

 the Gulf of Mexico, it exists in a broad sheet. 



It is to be expected that, extending as it does over so wide a space, it should present 

 very different characters in different localities. Accordingly, we find that much confu- 

 sion has existed in its classification. Mr. McClure, the first who attempted to syste- 

 matize American geology, considered it all as alluvial, but subsequent geologists soon 

 corrected this error. Mr. Conrad, in the first number of his "Fossil Shells of the 

 Tertiary of North America," divided it into the Upper Marine, Middle Tertiary, and 

 Lower Tertiary. In this arrangement, the Upper Marine corresponds very nearly with 

 Mr. LyelPs Pliocene, the Middle Tertiary comprises the Miocene and Eocene of the 

 same author, and the Lower Tertiary consists of sand and gravel containing Lignite 

 arid a few shells. Mr. Conrad has, more recently, adopted the classification of Lyell, 

 which is perhaps best, as the beds comprising his Lower Tertiary are hardly of sufficient 

 importance to constitute a separate formation. 



The latest, and probably, most correct views upon this subject are, however, those 

 which Mr. Conrad advances in the second number of the Bulletin of the National 

 Institution. He there supposes that we possess the Eocene, Miocene, and Post-Plio- 

 cene, as nearly as the parallel can be drawn between our strata and those of Europe. 

 Several links of the chain, however, appear to be wanting, as but one or two species 

 are common to the Eocene and Miocene, and only recent shells have been found exist- 

 ing in both the Miocene and Post-Pliocene. The Eocene does not possess a single 

 VOL. ix. 61 



