ART. 4 C AMEBIAN CONCHOSTRACA — ULRICH AND BASSLER 41 



WALCOTTELLA VENTROSA, new species 



Plate 5, Figures 4, 5 



Description. — Carapace broadly elliptical, truncated by the hinge 

 line; ends siibequal, curving regularly into the convex ventral mar- 

 gin; cardinal angles obtuse, almost rounded. Surface of valves 

 gently convex, the free edges narrowly rimmed. Main tubercle 

 rather small but very sharply elevated, located almost in the middle 

 of the antero-dorsal fourth. Midway between this and the cardinal 

 angle a minute ocular node is barely distinguishable. Surface of 

 shell minutely puncto-reticulate. 



Dimensions of figured specimen, a right valve: Length 3.1 mm., 

 height 2.2 mm., thickness about 1.0 mm. 



Remarks. — This pretty species doubtless is closely allied to W. 

 limatula. The principal difference lies in the outline which is more 

 uniformly curved, the ends consequently being nearly equal, whereas 

 in that species the posterior is the wider and the curvature of the 

 margins of the ends quite different. W. ventrosa seems also to be a 

 smaller species. 



Occurrence. — Associated with W. limatula and most of the other 

 species of the genus here described, in the Bright Angel shale, Bass 

 Canyon, Grand Canyon, Ariz. 



Holotype.— Cat. No. 56489, U.S.N.M. 



WALCOTTELLA SUBTRUNCATA, new species 



Plate 5, Figure 6 



Description. — Carapace subquadrate, hinge straight, ventral edge 

 moderately convex, ends, especially the anterior, obliquely truncated 

 so that the greatest length lies well beneath the mid height; postero- 

 ventral part of outline less abruptly curved than the corresponding 

 part on the anterior side. Surface moderately convex, the flattened 

 border narrow but well defined, the median tubercle of moderate size 

 but conspicuously elevated, located one fifth the length in front of 

 the middle of valve; two comparatively obscure nodes occur between 

 the main tubercle and the antero-cardinal angle. Surface of sheU 

 minutely reticulated. 



Dimensions of the type specimen: Greatest length 4.1 mm., 

 length of hinge 3.2 mm., greatest height 3.0 mm., thicloiess of single 

 (right) valve about 1.6 mm. 



Remarks. — T^''. subtruncata is easily distinguished from all the pre- 

 ceding species of the genus by its quadrate form. The following 

 W. longula alone resembles it in this respect. 



Occurrence. — Same as W. ventrosa, W. limatula, and other species 

 collected from the Bright Angel Shale at Bass Canyon, Grand Can- 

 yon, Ariz. 



Holotype. —C&t. No. 56490, U.S.N.M. 



