FREE INSTITUTE OF SCIENCE 



1555 



TERTIARY FAUNA OF FLORIDA 



Jacksonian and Vicksburg is known. At anj' rate, the proportion is less than 

 one-third that which unites the Chesapeake Miocene with the typical Pliocene. 



THE SHELL BLUFF GROUP. 



This supposed group was defined by Conrad, who placed it below the Jack- 

 sonian in the Eocene column, as forming the uppermost division of his Medial 

 Eocene, which included the horizons between the Chickasawan and the Jack- 

 sonian.* Later it was a subject of discussion, some error being generally 

 suspected. Professor W. B. Clark visited the locality and found above the 

 Ostrea georgiana bed a layer of shell limestone composed exclusively of the 

 valves of a species of Yoldia which had a more modern aspect than the strata 

 below and was noted by the writer as probably Eocene, f but showing indica- 

 tions of transition towards the Oligocene, then termed Older Miocene. In the 

 " Table of Tertiary Formations" compiled by the writer in 1895 a place was 

 doubtfully indicated for this portion of the Shell Blufif Group in the lower 

 Vicksburgian, it being stated at the time that its position was not definitely 

 settled.^ 



In 1900 the locality was visited and carefully examined by Mr. T. Wayland 

 Vaughan, of the United States Geological Survey, who determined the age of 

 the Shell Bluff formation below the Ostrea georgiana bed to be practically 

 identical with the Claibornian. The name of Shell Bluff is therefore to be ex- 

 punged from the column of groups of American Eocene, and relegated to the 

 rank of a mere local development of the Claibornian. Characteristic fossils 

 from Shell Bluff recorded by Mr. Vaughan are Venericardia planicosta and 

 alticostata, Corbula oniscus, Pteropsis lapidosa, Mesalia ohruta, Ostrea georgi- 

 ana, Turbinolia pharetra, and Endopachys Maclurei. Some forty species alto- 

 gether were collected. § 



In this connection it may be observed that the collection of typical speci- 

 mens of Ostrea georgiana Conrad shows that this species, to which large 

 oysters from all parts of the Tertiary have been hastily referred by some authors, 

 is well distinguished when in perfect condition by its surface sculpture. This 

 is characterized by low, flatfish, distally forked radial riblets, quite different 

 from anything observable on the surface of O. vtauriciensis or 0. virginica, 



* Checklist Inv. Fos. N. Am. Eocene, p. i, 1866. 



t Bull. U. S. Geol. Survey No. 84, p. 84, 1892. 



t Eighteenth Ann. Rep. U. S. Geol. Survey, pt. ii., p. 342. 



§ See Science, N. S., vol. xiii., p. 270, Feb. 15, 1901. 



