36 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



Remarks. — The ventral margin of the type specimen is imperfect. 

 As restored in our photographic ilhistration, it resembles W. pulchella, 

 but is still too elongate for that species. The antero-cardinal angle 

 also is different, while the median tubercle of the latter is more promi- 

 nent and pointed and situated farther anterior. Finally, W. pulchella 

 has smaller and closer punctae. W. scitula probably also is related 

 to W. apicalis, but the median tubercle of the latter is so much 

 stronger that confusion between them is very unlikely. Imperfect 

 specimens of W. oblonga might be confused, but complete examples 

 are readily distinguished by their oblong, equal-ended shape. 



Occurrence. — A single left valve only was found associated with the 

 preceding species in the Bright Angel shale at Bass Canyon, Grand 

 Canyon of the Colorado, Ariz. 



Holotype.—C&t. No. 56482, U.S.N.M. 



WALCOTTELLA PULCHELLA, new species 



Plate 4, Figures 11, 12 



Description. — Carapace egg-shaped, decidedly narrower in front 

 than behind, greatest height and greatest length, respectively, as 2 

 is to 3; hinge rather short, only five-eighths of the greatest length, the 

 two extremities obtusely angular and more nearly equal than usual; 

 valves gaping distinctly at the ends and probably also a little along 

 the ventral margin; edges of valves with a thin, wirelike rim. Sur- 

 face of valves rather strongly convex; flattening along free margins 

 inconspicuous; median tubercle small but prominent, pointed, its 

 apex located about one-tenth of the length of the carapace anterior 

 to the middle; eye node apparently not distinguishable. Surface of 

 shell very finely puncto-reticulate. 



Dimensions of type specimen: Length 4.0 mm., greatest height 2.7 

 mm.; greatest thickness, between apices of median tubercles, 2.3 mm. 



Remarks. — This neat species resembles W. ventrosa and W. limatula 

 but is easily recognized by its simple border, greater convexity, and 

 different outline. It is really more closely allied to W. leperditoides, 

 W. scitula, and W. apicalis. From all of these it is distinguished by 

 the liner pattern of its surface network. Compared further with 

 each, the first is larger and without a well-defined median tubercle, 

 the second is more elongate and has a less pointed node, while the 

 third has a larger tubercle and conical rather than convex valves. 



Occurrence. — The figured specimen only has been seen. This was 

 broken out of a block of the same bed of the sandstone in the Bright 

 Angel shale from which most of the species of this genus were procured 

 by Walcott at his locality in Bass Canyon, Grand Canyon of the 

 Colorado, Ariz. 



Holotype.— Cat. No. 56483, U.S.N.M. 



