TWO NEW MOLLUSKS OF THE GENERA OSTREA AND 

 EXOGYRA FROM THE AUSTIN CHALK, TEXAS 



By Lloyd W. Stephenson 

 Of the United States Geological Survey 



Two new species of Ostreidae recently found in the upper part of 

 the Austin chalk of central Texas are of interest because of their 

 restricted stratigraphic range, their known geographic range along 

 linear distances respectively of 60 and 75 miles, and the great num- 

 ber of individuals representing each species. One of them, Ostrea 

 centerensis, has been found at five localities along a line of outcrop 

 50 miles long, extending from Travis to Bell County, in a stratum a 

 foot or less in thickness, which, on Little Walnut Creek in Travis 

 County, lies about 25 feet below the top of the Austin chalk. The 

 other species, Exogyra tigrina^ occurs at its type locality on Little 

 Walnut Creek in Travis County, in a stratum of chalky marl a foot 

 or less in thickness, 35 feet below the top of the Austin chalk, 10 

 feet below the zone of Ostrea cenferensis, and has been found at 5 

 other localities, two of them farther south in Travis County, and 

 three toward the north, the most distant one being northeast of 

 Temple in Bell County. The linear distance along which this species 

 has been collected is 75 miles. 



The species, Exogyra tigrina^ is of especial interest because the 

 original color markings of the shell are still well preserved. This, 

 however, is not the first recorded species of this genus which exhibits 

 color markings, for similar markings appear on an Exogyra from 

 the upper Cenomanian and the lower Turonian of Europe, which 

 Coquand ^ in 1869 figured under the name Ostrea colwniba^ and de- 

 scribed in the text under the name Ostrea ratishonensis. He says: 

 " La surface, dans les exemplaires bien conserves, est couverte de 

 flammules brunes, obliques, en sautoir." The color markings on this 

 species, as represented in Plate 45, Figures 8 and 9, of the paper 

 cited, are very similar in character and distribution to those on 

 Exogyra tignna. 



1 Coquand, H., Monographie du Genre Ostrea, Terrain Cretace, p. 121, pi. 45, figs. 8-12, 

 1869. 



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