ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 85 



Dimensions. — Length =11.5 mm.; depth = 7 mm.; thickness of the 

 two valves together = 4 mm. (Cobbold, 1921.) 



Although apparently a valid species of Indiana, Leperditia lenti- 

 Jormis is unusual in the large size of its carapace. 



Occurrence. — Lower Cambrian, Olenellus limestone: Comley, 

 Shropshire, England. 



DIELYMELLA, new genus 



Description. — Carapace bivalved, 5 to 10 mm. long; outlines pod 

 shaped in lateral view, acutely elliptical in edge views, thickest 

 in the anterior half; valves not closing tightly, leaving a narrow 

 gap that extends from the anterior extremity of the hinge around the 

 ventral edge and through the postdorsal region; gap usually narrowest 

 in the ventral part and widest about the middle of the posterior side; 

 anterior extremity truncated, forming an angle of 90° or less with the 

 hinge line; the antero-dorsal extremity generally beaklike, suggesting 

 the rostrum of a pelecypod but not projecting above the hinge line; 

 posterior outline strongly rounded, ventral edge more gently convex; 

 hinge line long, straight. Surface of valves moderately convex, slight 

 posteriorly but gradually rising toward the umbo. Test thin, in 

 color and probably chemical composition like that of a linguloid 

 brachiopod; surface smooth or finely punctate. Interior of valve 

 with a minute elevation in the rostral cavity which is indicated in the 

 cast by a corresponding depression. Usually an obscure fold extends 

 downward from this elevation and parallels the anterior margin. 



Genotype. — Dielymella recticardinalis, new species. 



Like Walcottella, most of the known species of this new genus of 

 bivalved Crustacea occur in the Bright Angel shale of the Grand 

 Canyon. While the species of the two genera are not all associated, it 

 is 3^et a fact that common occurrences prove them all to belong to 

 essentially the same bed. Only D. recticardinalis is confined to a 

 single locality at which no species of Walcottella has been recognized, 

 but with it occur D. appressa and D. nasuta, both of which seem to be 

 abimdant at the Bass Canyon locality from which most of the Wal- 

 cottellas were procured. 



Respecting the systematic position of Dielymella, it is thought to be 

 with Indiana Matthew on the one hand, and an unquestioned Phyllo- 

 carida like the Ordovician Caryocaris and the Devonian Elymocaris, 

 on the other. Of course there is a wide structural interval between 

 these two extremes, but D. recticardinalis, especially the variety 

 angustata, seems to fairly represent an intermediate stage. These 

 Cambrian bivalved Crustacea are in a way synthetic, their modifica- 

 tions tending generally in two directions, one toward the leperditoid 

 Ostracoda, the other toward the ceratiocarid Phyllopoda. Dielymella 

 illustrates a modification toward the latter type, while Indiana and 



