149 St. Maurice and Claiborne Pelecypoda 149 



gse are sharp or lamellar, while in bastropensis they are rounded, 

 or sem'icylindrical. The pallial sinus deeper than in trigoniata. 



Type specimens. — PI. 47, fig. 4, Mus. No. 407, Sta. 37; fig. 6, 

 No. 1721, Sta. II. Texas State Museum. Now at the Univ. 

 of Texas. 



Geological horizon. — St. Maurice Eocene. 



Localities of form No. i. — Devil's Eye, Colorado River, Bas- 

 trop Co. ; Smithville, Bastrop Co. ; Mosley's Ferry, Brazos R., 

 Burl. Co. ; Brazos R., 500 3'ards below the mouth of Little Bra- 

 zos R., Brazos Co. ; Cedar Cr., S. E. corner of Wheelock 

 league, Robertson Co. ; Campbell Cr., Gifford headright, Rob- 

 ertson Co. ; Orrell's Crossing, Elm Cr., Lee Co. 



Localities of form No. 2. — Rio Grande, six miles above Starr- 

 Zapata Co. line ; Rio Grande, Mexican side, one mile above the 

 Starr-Zapata Co. line ; Rio Grande, 15 miles below Carrizo ; Rio 

 Grande, 6 miles below Carrizo ; Dr. Williams' quarry, on Rob- 

 ert Stephenson's headright, Brazos Co. ; Hammett's Branch, 

 Louisiana. 



Meretrix subcrassa Lea, PI. 46, Fig. 7 



Cytherea subcrassa h^a, Cont. to Geol., 1833, p. 67, pi. 2, fig. 43. 



Lea's original description. — Shell somewhat inflated subtriangular, 

 concentrically striate, thick about the region of the hinge ; substance of the 

 shell thick : beaks thick and elevated ; lunule cordate, indistinct ; teeth 

 rather elevated and compressed ; excavation of the pallial impression rather 

 small and roundish ; cavity of the shell deep and rounded ; margin crenu- 

 late. 



Diam Length i.i. Breadth 1.2 of an inch. 



Observations. — This shell most resembles trigoniata herein described. 

 It is like that shell in outline, but it is thicker, and differs also in having a 

 crenulated margin. 



We have found nothing among our Claiborne sand collec- 

 tions showing the extreme thickening of the shell matter shown 

 by Lea's specimens. But we are inclined to believe that when a 

 large enough series of trigoniata from the "sand" has been col- 

 lected, this so-called species will be found to grade imperceptibl}" 

 into trigo7iiata. Lea evidently did not notice that some trigoni- 

 ata show signs of marginal crenulation. Conrad's discoidalis (pi. 



