120 University of Texas Bulletin 



NUCULA WENOENSIS n. p. 

 PI. 10, figs. 10-11 



MEASUREMENTS: I (type) II III 



Breadth 8.5 10.2 13.0 



Height 14.9 17.5 18.5 



Length 20.5 23.7 26.0 



HORIZON : Weno formation, blue shale, Red River region, occasional ; 

 clay ironstone, occasional. 



LOCALITY: 604, Frisco cut, three-fourths mile north of Union Sta- 

 tion, Denison, Texas (type locality) ; 601, pit of brickyard, one and three- 

 fourth miles southeast of Gainesville, Texas. 



DESCRIPTION: Shell sub-equivalved, moderately inflated, inequi- 

 lateral, beaks placed back of the middle of the valve. Ventral margin a 

 long, sub-elliptical curve, sharply angulated posteriorly, more rounded an- 

 teriorly. Anterior margin long, slightly convexed, beaks opposite, approx- 

 imate; posterior margin shorter than anterior, nearly straight, lacking 

 a prominent incision just posterior to the beaks. Anterior areas cres- 

 centic, together forming an elongate biconvex strip, posterior areas form- 

 ing a strip proportionately shorter but equally wide. These are crossed 

 by plain recurved growth lines. 



Valves nearly smooth in appearance; ornamentation consists of numer- 

 ous fine. simple radial s^riaeiform ridges, crossed by growth lines and 

 growth rings. These ridges on reaching the ventral border form a slightly 

 incised, non-crenulate edge. Hinge taxodont, essentially as in Nucula 

 nokonis. 



This species differs from N. hokonis in its form and ornamentation. 

 The anterior margin 'is straighter and longer; the posterior margin is 

 straighter and lacks the sharp notch just back of the beaks, having in- 

 stead a gentle curvature ; the posterior "areas" are much more pronounced 

 in side view, and instead of lying flat are elevated at the line of junction 

 of the valves by a posterior projecting carina. The ornamentation at once 

 distinguishes the two species, for in N. nokonis the radial ribs are very- 

 pronounced and the concentric growth lines on crossing them form a 

 system of prominent squares ; the concentric ribs on approaching the an- 

 terior border become zigzag or wavy making a conspicuous V-shaped an- 

 gulation, and in the same region the concentric ribs become crenulate and 

 lamellated. The radial ribs form a prominent scalloped edge at the ven- 

 tral margin of the valve. 



