ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 89 



cardinal region. The surface of test also is less distinctly punctate 

 or quite smooth, though this difference is not usually apparent, the 

 shell being usually exfoliated. Young specimens resemble D. dor- 

 salis, but the posterior margin in more uniformly curved. D. nasuta 

 is more convex and its anterior extremity quite different in shape. 



Occurrence. — Bright Angel shale; three specimens found associated 

 with D. recticardinalis at head of Nunkoweap Valley, seven specimens 

 at the Bass Canyon locality with numerous examples of Walcottella; 

 Grand Canyon of the Colorado, Ariz. 



Holotype.— Cat. No. 56506, U.S.N.M. 



DIELYMELLA DORSALIS, new species 



Plate 10, Figure 1 



Description. — Carapace oblong, subquadrate, the ends nearly 

 equal, the posterior slightly the wider and its outline most prominent 

 in the dorsal half; hinge line very long, terminating abruptly at the 

 rectangular front, postcardinal angle sharply rounded; ventral half of 

 outline semielliptical. Surface of valves moderately convex with an 

 obscure flattening in the mesial third of the dorsal half. An obscure 

 elevation or node occurs near the antero-dorsal angle in the type 

 specimen; but as it is slightly injured at this point, it is not possible to 

 decide whether the node is a natural feature or accidental. Test 

 somewhat exfoliated, apparently smooth. 



Dimensions of a right valve: Greatest length, 4.2 mm.; height, 

 3.0 mm.; thickness, about 0.75 mm. 



Remarks. — Unfortunately the unique example on which this species 

 is founded is not quite perfect. As interpreted it is a right valve. 

 Whether a right or a left, it is certain that the postdorsal part of its 

 outline is more abruptly rounded and more produced than in any of 

 the other species referred to this genus. Except in this respect, 

 D. dorsalis is snfRcientlj^ like young examples of D. appressa to 

 necessitate care in their discrimination. The flattening of the 

 centrodorsal region, on the other hand, is so suggestive of the associ- 

 ated Indiana impressa that it is difficult to distinguish specimens 

 with imperfect margins. Good specimens, of course, offer no difficul- 

 ties, the dorsal half of the outline being quite different in the two 

 species. 



Occurrence.- — Bright Angel shale, Bass Canyon, Grand Canyon, Ariz. 



Holotype. —Cht. No. 56505, U.S.N.M. 



DIELYMELLA BREVIS, new species 



Plate 10, Figures 12, 13 



Nothozoef vermoniana (part) Walcott, 10th Ann. Rep. U. S. Geol. Surv., 1890, 



pi. 80, figs. 4a, 46 (not fig. 4). 

 Not Nothozoe ver7JW7i(ana Whitfield, Bull. Amer. Mus. Nat. Hist., vol. 1, 1884, 



p. 414, pi. 14, figs. 14, 15. 



