145 St. Maurice and Claiborne Pelecypoda 145 



length and height equal ; buccal end acutely rounded ; anal end more ob- 

 tuse ; summit prominent. 



A small Eocene species, with the external surface somewhat worn. It 

 appears to have had concentric lines. 



Locality. — Western Texas. 



Nothing in our collection has thus far appeared which 

 could be referred to this small species. Conrad omits it in his 

 Catalogue of Eocene Testacea etc., and Heilprin has overlooked 

 it in his "Eocene Mollusca of the State of Texas". The type 

 specimen, which should be in the collection of the U. S. Nat. 

 Mus., is mislaid or lost. The figure herewith given is after 

 Conrad. 



Trigoniata stock 



From the Midway ripleyana up through the Jackson stage 

 there have appeared representatives of this stock, showing pretty 

 generally a tendencj^ toward a triangular outline, fine and sharp- 

 1}' incised grovvth-lines and a long and comparatively narrow 

 lunule (though the specimens are sometimes very much inflated.) 



Perhaps the most characteristic feature of this stock is the 

 tendency to show indications of radiate markings and marginal 

 crenulation. The former are often indistinctly shown on the 

 anterior portion of typical trigoniata from Claiborne, while sug- 

 gestions of marginal crenulation along the anterior basal margin 

 are not rare. These features seem to be rather characteristic of 

 the thicker, perhaps diseased forms. 



Hatchetigbeensis is unusually circular in outline and the 

 growth-lines tend to concentration, or marginal corrugations. 



In the St. Maurice beds there are various close approaches 

 toward typical trigoniata. The bastropensis of our old MS Texas 

 report is somewhat more circular. While in winnensis the out- 

 line is roughly triangular, the surface seems usually to .show 

 signs of radiating threads, fine marginal crenulations and less 

 even sculpturing concentrically. A variety of this, in which no 

 radii have been seen is excessively globose with microscopic con- 

 centric lines on the umbones but with wave-like undulations be- 



