2 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 76 



The section on Little Walnut Creek described below shows the 

 stratigraphic position of each of the new species; the section was 

 not accurately measured and the thicknesses given are only approxi- 

 mately correct. 



Section on Little Walnut Creek in the vicinity of the iron bridge at the crossing 



of the old Sprinkle road, Travis County 

 Taylor marl : Feet 



Marly clay, poorly exposed 10 



Unconformity? 

 Austin chalk : 



Alternating layers of rather massive marly chalk and harder chalk; 

 contains Exogyra ponderosa var. erraticostata Stephenson, and Ostrea 



species (aff. 0. diluviana Linnaeus) within 10 feet of top 20 



Rather soft argillaceous, marly chalk ; a one-foot layer, 5 feet below^ the 

 top contains many specimens of the new oyster, Ostrea centerensis 



Stephenson 15 



Massive chalk with several softer marly layers ; the upper one foot of 

 chalk contains great numbers of the new oyster, Exogyra t-igrina 

 Stephenson, and just below this is a layer containing many shells of 

 Exogyra laoviusciila Roemer (a varietal form) ; Exogyra ponderosa 

 Roemer occurs in the lower part of the section 15 



The type locality of Ostrea centerensis is a ravine north of the 

 public road, 2% miles west-northwest of Sparks, % mile north of 

 Center Lake School, Bell County. In this ravine marly chalk, ex- 

 posed downstream for about 100 yards, contains considerable numbers 

 of this new oyster; the marly bed ends against a small fault whose 

 downthrow is toward the road, and a chalk bed exposed on the north 

 or upthrow side of the fault contains the shells of Exogyra tigrina 

 Stephenson. Doubtless the latter bed belongs stratigraphically be- 

 low the former, as it does in the section on Walnut Creek. 



The photographs (pis. 1-3) were made by W. O. Hazard, in the 

 j)hotographic laboratory of the United States Geological Survey, and 

 were retouched by Frances Wieser, also of the Geological Survey. 



OSTREA CENTERENSIS, new species 



Plates 1 and 2 



Description. — Shell of medium size, inequivalve, subovate some- 

 what elongate in outline, generally curving more or less strongly 

 backward, though nearly straight in some individuals ; valves flattish, 

 close-fitting with little room for the soft parts when tightly closed; 

 shell wall thin to moderately thick, but tough and strong; the shell 

 shows a tendency to the development of wing-like anterior and pos- 

 terior projections in the dorsal portion, but this is a variable feature. 

 Dimensions of the type, a nearly complete individual with both valves 

 preserved: Length 70 mm., height 68 mm., thickness 12 mm. Di- 

 mensions of a large left valve (pi. 2, figs. 1, 2) : Length 83 mm., 

 height 85 mm., convexity 10 mm. 



