1849.] MOORE ON TERTIARY BEDS IN SAN DOMINGO. 43 



Mr. Conrad* has lately described a formation in South Carolina, 

 in the bluffs of the Mississippi, which is distinctly newer than the 

 eocene beds of Claiborne. Out of 177 shells, it contains only two 

 which are common to it and the subjacent eocene, while not a single 

 species is identical with existing forms. In the upper part of it are 

 fomid the teeth of the Carcharodon megalodon. To this formation 

 Mr. Conrad has provisionally given the name of upper eocene. Two 

 of the shells are undistinguishable, by their engravings and descriptions, 

 from two of the San Domingo shells, and a great many are closely 

 allied. Should it prove on further examination that the San Do- 

 mingo beds are referable to two formations, the recent species being 

 principally confined to the upper and the extinct to the lower, I think 

 it probable that the lower would be found nearly related to those 

 so-called upper eocene beds in South Carolina. 



There is a character attached to a portion of this collection which 

 is too remarkable to leave unnoticed. Mr. Sowerby was much struck 

 with the resemblance of many of the shells to recent species inhabiting 

 the seas of China, Australia, and even the western coast of iVmerica ; 

 a resemblance so close, that that naturalist hesitated before pronoun- 

 cing them to be distinct ; whilst he identifies without any doubt two 

 of the shells with the recent Venus puerpera (Linn.) of the Indian 

 Ocean, and the Phos Veraguensis which was dredged up in the Bay 

 of Veragua by Mr. Hinds, during the Voyage of the Sulphur. It is 

 certainly remarkable to find a shell living in the same latitude with 

 its fossil analogue, and separated from it only by a narrow isthmus, 

 when it is recollected upon what grounds the high antiquity of the 

 division of the Atlantic and Pacific Oceans into distinct faunas has 

 been established. M. D'Orbigny has shown that the tertiary beds 

 which flank the two sides of the Cordilleras do not contain one species 

 in common. Mr. Conrad has lately published a list of all the recent 

 shells known to him which are found in the American miocene, 49 in 

 number, which without an exception are Atlantic species. Sir C. 

 Lyell has shown that most of the recent shells found in the miocene 

 of the United States are confined to the western shores of the At- 

 lantic. To this there is one exception, the Calyptrcea costata, which 

 is found living at Valparaiso and fossil in American miocene beds. 

 It must be remembered that M. D'Orbigny's conclusions are formed 

 from shells derived from latitudes 31° to 40° S. ; and the most 

 southern of the North American miocene beds are in lat. 33° N. : 

 so that a channel or sound may have existed in the equatorial parts 

 during some portion of the tertiary period, by which some few of the 

 tropical shells may have migrated from the one ocean to the other, 

 while those living 30° to the north or south of the line would be as 

 effectually separated as those actually living in the two oceans 30° 

 north of the Straits of Magellan. It should be borne in mind that 

 the Isthmus of Panama is not merely narrow, but low land : the con- 

 tinuity of the Andes (I quote Mr. Hinds' words) is here quite broken ; 

 and instead of the tableland north of Nicaragua of the height of 4500 



* American Journal of Science, 1846> New Series, i. 



