88 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



appreciably from pressure, the suggested possible cause of the dif- 

 ferences is discredited. 



Occurrence.- — Same as the preceding. 



Holotype.-— Cat. No. 56511, U.S.N.M. 



DIELYMELLA NASUTA, new species 



Plate 10, Figures 10, 11 



Description. — Carapace somewhat obliquely acuminate ovate, the 

 greatest height equaling about two-thirds the length; hinge straight, 

 long, its extremities sharply defined, the posterior angle about 120°, 

 the anterior less than 90°, prominent, obtusely claw-shaped; anterior 

 end narrower than the posterior, straight or very gently concave in the 

 upper half; the remainder of outline, from middle of anterior side to 

 posterior extremity of hinge, a nearly regular elliptical curve, the 

 narrowest part of which corresponds to the postventral margin. 

 Valves strongly convex, attaining greatest thickness in anterior half; 

 no border. Surface of shell usually appearing smooth but when well 

 preserved exhibits minute and irregularly arranged punctae. 



Dimensions of best specimen, a left valve, apparently of average 

 size: Greatest horizontal length, 7.7 mm., length diagonally across 

 valve from beak to postventral margin, 8.3 mm.; greatest height 

 (just behind middle) 5.9 mm.; greatest thickness, single valve, about 

 2.7 mm. 



Remarks. — In this species the anterior extremity is even more 

 acuminate and beaklike than in D. recticardinalis . It differs from that 

 species further in being proportionately much wider posteriorly and 

 narrower anteriorly, in the more definite angularity of the postcardinal 

 margin, and in being relatively shorter. All these differences apply 

 again in comparing it with D. appressa, with the additional fact that 

 the latter is less convex. 



Occurrence. — Bright Angel shale. Bass Canyon and head of 

 Nunkoweap Valley, Grand Canyon of the Colorado, Ariz. 



Cotypes.—C&t. Nos. 56508, 56509, U.S.N.M. 



DIELYMELLA APPRESSA, new species 



Plate 10, Figure 9 



Description. — Disregarding the truncation of the hinge line and the 

 rectangular antero-cardinal region, the outline is elongate elliptical. 

 The valves are compressed convex, the middle third of the profile, in 

 a dorsal view, almost straight. Surface of test obscurely punctate 

 or smooth; irregularly striated along the free margin. 



Dimensions of a large carapace: Greatest length, 9.4 mm.; greatest 

 height, 5.5 mm.; greatest thickness, 2.3 to 3.0 mm. 



Remarks. — This species is easily separated from D. recticardinalis 

 being less convex, wider posteriorly, and less prominent in the antero- 



