PALEONTOLOGY ZORRITOS FORMATION 97 



Shell of moderate size, high, trigonal, marginally cuneate 

 in cross-section. Anterior margin rounded, descending 

 abruptly into the ventral margin, which continues to descend 

 almost to the posterior extremity of the shell, whence the 

 posterior margin rises abruptly in an elliptical curve. Um- 

 bones erect, submedian in position, slightly prosogyrate, 

 inflated. Anterior surface smoothly convex, the valves 

 meeting at an acute angle ; median surface convex ; poster- 

 ior surface somewhat excavated behind the beaks, with a 

 very gentle carina descending from the beaks and flattening 

 towards the posterior marginal angle. At the marginal 

 juncture of the valves the surfaces are somewhat attenuated, 

 the form being wedge-like. Sculpture of 25 low, squarish 

 ribs, marginally nodulose over the entire length of the left 

 valve, on the right valve slightly nodulose anteriorly, smooth 

 posteriorly ; upper sculpture of both valves unknown. Inter- 

 spaces slightly wider than ribs anteriorly, equal mesially 

 and posteriorly, crossed by fine concentric lines. Area 

 lanceolate, smooth, bounded by a distinct, fine marginal 

 furrow. Teeth, visible in cross-section as joined valves are 

 viewed from above, fine mesially, coarser distally. Inner 

 shell unknown. Length, 16.25 ; height, 15.5 ; diam., 12.5 mm. 

 A single complete specimen of A. zorritensis, in Nelson's 

 collection, was marked by him "species indeterminate," but 

 inasmuch as all of the characters of the outer shell are visible 

 save the upper sculpture of the ribs it is deemed advisable to 

 describe the form. Unfortunately, the exact geologic hori- 

 zon of Nelson's fossils is not known, and description of them 

 thus normally carries but little significance ; but in the case of 

 a clearly defined form such as this it is well to name the 

 species, trusting to future collecting in the region for its 

 geologic allocation. It is fairly safe to say that it was taken 

 from the Zorritos formation, but inasmuch as Nelson's forms 

 appear to have come indiscriminately from all three units of 

 that group a more definite assumption is not warranted. It 



