124 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



Plate 10 



The specimens represented in Figures 1-11 are from sandstones in the Bright 

 Angel shale, Middle Cambrian, in the Grand Canyon, Ariz. 



Page 

 Figure 1 . Dielymella dorsalis, new species 89 



The unique specimen, a right valve, X 4, on which this 

 species is based. It is defective at the antero-dorsal angle 

 and the indicated small node in this part is probably 

 more strongly defined in the photograph than it should 

 be. 

 2. Indiana impressa, new species 81 



A left valve of this species, X 4, slaowing the general out- 

 line and the characteristic broad median depression of 

 the surface. In the right valve the dorsal angles are less 

 obtuse, and this valve probably overlaps the left at 

 these points. 

 3-7. Dielymella recticardinalis, new species 8& 



3. A large and typical right valve, X 3. 



4. Another large riglit valve, X 3, varying somewhat 



from the average form in the greater and more uni- 

 form convexity of the antero-ventral half of the out- 

 line. 

 5-7. The left side of one (6) and the right sides (5 and 7) 

 of two other entire examples, X 4. These specimens 

 are smaller than the preceding two. In fact Figures 

 7, 6, 5, and 3 illustrate a progressive series in the 

 matter of size, and prove that the variation in the 

 specific characters of the valves is practically negligible 

 in the later stages of growth. 



8. Dielymella recticardinalis angustata, new variety 87 



An example so much narrower than the usual form of the 

 species that it is thought desirable to distinguish it as 

 a distinct variety, X 4. 



9. Dielymella appressa, new species 88 



Right side of a nearly perfect cast of the interior, X 3, 

 showing the shorter and less convex form of the car- 

 apace that distinguishes this species from /. recticard- 

 inalis. 

 10-11. Dielymella nasuta, new species 8S 



10. A good left valve, slightly imperfect along the pos- 



terior edge yet giving a fair idea of the peculiar- 

 ities of the species, X 3. The prominence of the 

 anterior extremity of the dorsal edge and the 

 unusual width of the posterior half are the most 

 important features. 



11. A specimen retaining the two valves, X 3. In 



photographing it was posed so that neither valve 

 shows the full width. The depressions in the 

 surface are due to grains of the quartz sandstone 

 matrix pressed into the flexible test. 



