ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 35 



guishable. Surface of shell distinctly puncto-reticulate, the pattern as 

 coarse as in any and obviously coarser than in most of the other species. 

 No well-defined flattened marginal border. 



Dimensions of the best specimen, a left valve: Greatest length, 

 4?5 mm.; greatest height, 3.5 mm.; thickness (one valve), about 

 1.4 mm. 



Remarks. — In deciding that the smaller of the two ends is the 

 anterior in this species, the judgment of the writers has been based 

 chiefly on the fact that in nearly all bivalved Crustacea having a 

 straight hinge, the antero-cardinal angle is sharper than the posterior; 

 and when the outline is oblique with respect to the hinge line, the 

 "swing" is backward. Relying on these criteria, the median tubercle 

 is situated behind the middle of the valves, an arrangement that 

 serves well in distinguishing the species from all the others now 

 referred to the genus. Aside from this peculiarity, W. breviuscula 

 resembles W. pulchella, the principal other difterences being that the 

 ends are less unequal in width and the surface ornament coarser in 

 the present species. In W. apicalis the apex of the median tubercle 

 is slightly in front of the middle, low ridges are recognizable in the 

 anterio-cardinal region, and the slope of the surface of the valves 

 from the apex to the margins more suggestive of the uniform grade 

 of a depressed cone. 



Occurrence. — Bright Angel shale, Bass Canyon, Grand Canyon of 

 the Colorado, Ariz. Two specimens only found. Associated with 

 the preceding. 



CotT/pes. —C&t. No. 56481, U.S.N.M. 



WALCOTTELLA SCITULA, new species 



Plate 4, Figure 7 



Description. — Carapace somewhat elongate, obliquely subovate, 

 with long, straight hinge, beginning rather abruptly at the anterior 

 end but passing much more gradually into the semicircular curve of 

 the posterior margin; anterior end considerably narrower than the 

 posterior. Surface of valves rather strongly convex, rising from the 

 margins with first a convex, then a nearly uniform depressed conical 

 slope to the apex. The summit is located a little in front of the mid- 

 dle, does not rise abruptly in profile, but is marked by a smooth space. 

 Ventral part of margin not preserved, but that remaining on the ends 

 seems not to have been provided with a flattened border. Ocular 

 node not developed. Surface of shell puncto-reticulate, the pattern 

 in the median area tending to a concentric arrangement of the punctae 

 about the smooth apical spot. 



Dimensions of the unique type specimen: Greatest length, 4.7 mm.; 

 greatest height, about 3.0 mm.; greatest thickness, left valve only, 

 about 1.2 mm. 



