PALEONTOLOGY ZORRITOS FORMATION 175 



dorsal margin straight in left valve, biangular in right. Um- 

 bones anterior, not prominent, sharp, slightly prosogyrate. 

 Anterior surface rounded. Surface bounded by anterior 

 margin, first posterior carina, and ventral margin forms a 

 triangle, subcylindrical in contour. First posterior carina 

 prominent, bevelled in both valves, curved upward ventrally 

 and leading into the umbo in a straight line in the left valve, 

 curving over slightly in the right valve on account of its 

 convexity. Second posterior carina not prominent ; clearer 

 in right valve than in left. Intercarinal space concave in 

 both valves, larger in right. Sculpture of raised concentric 

 ridges, whose sharper slope is above, about 1 1 in the space of 

 5 mm. on the right valve and 16 in the same space on the 

 left. The sculpture is of constant strength over the entire 

 shell. Neither lunule nor escutcheon is apparent. Interior 

 of shell unknown. The shell is of greatest diameter at the 

 prominent carinae. Length, 11.5; height, 7.5; diameter, 

 4 5 mm. 



This species is one of a series of closely related Cuneocor- 

 bulas in the Zorritos, being clearly similar to C. fabiformis, 

 C. propinqua, and, to a lesser degree, to C. acutirostra. One 

 specimen only occurs in the collection. The characteristic 

 sculpture and bevelled carina of C. bravoana are undoubtedly 

 features of a species distinct from those named, despite the 

 fact that other external characters are close enough to cause 

 some hesitation in pronouncing it different. It differs from 

 C. fabiformis in sculpture, as suggested above, in having 

 more prominent concentric rugae, which are less curved 

 mesially than in fabiformis; the bevelled carina is lacking in 

 fabiformis, and the general shape of bravoana is nearer to 

 the strictly triangular. C. propinqua is nearer generally. 

 However, the lack of the bevelled carina, the finer and more 

 irregular sculpture, the broader ventral surface, and the 

 more sharply ascending ventral margin of propinqua mark it 

 as distinct. C. bravoana does not have the chisel-like point 

 of propinqua. 



