34 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



In its outline this species resembles W. hreviuscula and W. leperdi- 

 toides, but is distinguished at once by the much greater strength and 

 prominence of its median tubercle. 



Occurrence. — Middle Cambrian, sandstone beds lq the Bright Angel 

 shale just above the massive lower bed, near mouth of Bass Canyon, 

 K-wagunt Valley, and Chuar Valley, Grand Canyon, Ariz. 



Cotypes. —C&t. Nos. 56477, 56478, U.S.N.M. 



WALCOTTELLA CONCENTRICA. new species 



Plate 4, Figures 3, 4 



Description. — Carapace comparatively small, broadly subovate, the 

 outline, except for the short and not sharply defined hinge line, 

 approximately circular. As a rule one side, thought to be posterior, 

 is somewhat narrower and longer than the other. Surface of valves 

 somewhat depressed convex to the base of the but slightly eccentric, 

 strong conical tubercle; or the whole surface may slope away from 

 the apex with little change in angle. The specimens present no indica- 

 tion of smaller anterior nodes, nor is there any satisfactory evidence 

 of a flattened border. Surface of shell rather coarsely punctate, the 

 punctae elongate and so arranged that they suggest interrupted 

 concentric wrinkles. 



Dimensions of an average carapace: Length 3.0 mm.; greatest 

 height 2.5 mm.; thickness between apices of median tubercles about 

 2.5 mm. 



Remarks. — This species, besides being smaller than the others is 

 readily distinguished by its nearly circular valves and the concentrical- 

 ly arranged and wrinklelike pitting of the surface. When exfohated 

 it is not so easily separated from the young of W. apicalis, to which 

 it is doubtless also closely alhed. The collection contains eight 

 specimens. 



Occurrence. — Bright Angel shale, Bass Canyon and Chuar Valley 

 Grand Canyon of the Colorado, Ariz. Associated with the preceding. 



Cotypes.— C&t. Nos. 56479, 56480, U.S.N.M. 



WALCOTTELLA BREVIUSCULA, new species 



Plate 4, Figures 5, 6 



Description. — Carapace of medium size, subovate; hinge line two- 

 thirds the greatest length, straight, the anterior extremity sharply 

 angular, the posterior angle more obtuse; ends nearly equal, the 

 anterior slightly the narrower and less curved in the upper part of its 

 outline. Valves moderately convex, the slope of the rounded surface 

 clearly distinguished from the base of the conical median tubercle. 

 This, though a prominent feature, is smaller than m W. apicalis and 

 W. concentrica, and is situated well behind the center, provided the 

 smaller end is really the anterior. Ocular and other nodes indistin- 



