118 University of Texas Bulletin 



PELECYPODA 



NUCULA NOKONIS n. sp. 



PI. 10, figs. 12-16, 19-20 



MEASUREMENTS : I 



Length 17.0 mm. 



Height 13.4 mm. 



Thickness 8.3 mm. 



HORIZON : Blue shale and red clay-ironstones of upper half of Weno 

 formation, Red River region. 



LOCALITY : 601, pit of brickyards, one and three-fourths miles south- 

 east of Gainesville, Texas (type locality) ; 604, cut of Frisco track, three- 

 fourths mile north of Union Station, Denison, Texas. 



DESCRIPTION : Shell inequilateral, sub-equivalved, 'sub-triangular in 

 contour, with fine radial ribs. 



Right valve, exterior: Beak sharply rounded, angle 100 degrees; a 

 straight rounded ridge runs from the umbo to the anterior angle, making 

 with the anterior margin a long low triangle* whose other two sides are 

 almost straight portions of the anterior margin. These portions are in 

 length as two to one, the dorsalmost portion being the lunger, and make 

 with each other an angle of 15 degrees. The ventralward portion, if ex- 

 tended, would make with the extended ventral side an anterior angle of 

 60 degrees; the actual angle is sharply rounded. The ventral margin is 

 gently convex downward, and extends backward to the sharply rounded 

 posterior angle (90 degrees). The postero-dorsal margin is straight. 



The ornamentation consists of about 47 very flat low radial ribs which 

 are unbranched and continuous from beak to ventral margin. Between 

 these lie fine straight depressions one-sixth to one-eighth the width of a 

 rib. The ribs are crossed by fine scalloped growth lines with the convex- 

 ities pointed ventrally, giving to the growth lines a fine crenulate appear- 

 ance. On the anterior area (lunule) the ribs disappear and the growth 

 lines thicken slightly. This area contains a radial anterior ridge and a 

 concurrent posterior depression ; the growth lines in crossing these are 

 crenulate. TWO deeper concentric constrictions representing stages of 

 growth lie in the ventral half of the valve. 



On the shorter posterior area (escutcheon) the growth lines turn dor- 

 sally and passing over a radial depression and a radial ridge, converge to 

 the margin. 



Right valve, interior: The rounded, posteriorly directed beak is slightly 

 elevated over the posterior margin. This margin is gently convex over 



