22 University of Texas Bulletin 



depression on the posterior side near the spirally coiled 

 beak, the point of which is pressed down against the sur- 

 face of the shell. Sometimes this valve is entirely oval, 

 especially in immature individuals ; in other specimens it is 

 distinctly spirally coiled, developing a relatively high um- 

 bonal ridge. 



The ornamentation of the surface consists only of spiral 

 lines of growth and lamellae. Only the superior margin 

 of the umbonal region is finely crenulated. Traces of fine 

 crenulation show sometimes on the inferior margin. 



The interior of the lower valve does not change mater* 

 ially from the immature stage to the well developed spec- 

 imen. The ligamental groove is always very narrow and 

 deep, spirally coiled, and follows the superior margin. A 

 faintly crenulated ridge is below it, the margin on the upper 

 half of the cavity being always slightly crenulated. 



The cavity continues into the beak, more or less. 



The muscular impression is near the posterior side and 

 close to the region of the beak. 



The interior of the upper valve does not change materially 

 from the immature stages to the final development of the 

 species. The ligamental groove is very long, very narrow 

 and deep, spirally coiled, follows the superior margin of the 

 valve, and obliquely crosses the spiral lamellae of growth. 

 The space between the groove and the superior margin is 

 finely crenulated. 



The muscular impression is strong and exactly under the 

 umbo, as has already been observed by Roemer. 



Occurrence : 

 Del Rio clay. 



Locality of the specimens studied: 

 Shoal Greek, Austin, Texas. 



