PALEONTOLOGY ZORRITOS FORMATION 109 



Area (Scapharca) chamnensis n. sp. 

 PLATE V Fig. 15. 



Shell very inequilateral, moderately inflated, produced 

 posteriorly. Anterior margin rounded, meeting hinge-line 

 at approximately a right angle ; ventral margin slightly 

 curved, descending posteriorly to a point about 12 mm. 

 beyond the vertical bisectrix of the beak, whence it curves 

 upward to form the posterior margin, which is broadly 

 rounded, and meets the hinge-line at an angle somewhat 

 greater than 90. Umbones conical, situated at about the 

 anterior third of the full length, pointed, with the upper 

 surface sloping anteriorly; umbonal slope characterized by 

 distinct but gentle furrow about 3 mm. wide, bounded by an 

 anterior and a posterior keel, both of which die out distally, 

 the anterior surface being gently hollowed out, and the 

 posterior keel flattening out into the alate posterior surface. 

 Ribs, probably 30 in number (36 visible in specimen), very 

 narrow in middle of shell, with interspaces 2-3 times as 

 broad ; broader and flat-topped on posterior part of shell and 

 more closely spaced anteriorly. Area and teeth not visible. 

 Length, 33.5; height, 23; (semi) diameter (estimated) 7-8 

 mm. Locality : West side of hill between Quebradas Heath 

 and Charon, Zorritos district. Horizon : Lower Zorritos. 

 Collection : Johns Hopkins University. 



This form is represented by a single indifferently pre- 

 served pseudomorph, all of whose characters are not visible, 

 but of which the observable features are sufficiently distinct 

 from the most closely related described forms to warrant the 

 description of the species. It is most similar to A. gatun- 

 etmj'Toula, 108 from the Gatun beds of Panama, but differs 

 in that its posterior margin is truncate, not rounded ; there 

 is no tendency toward bifurcation of the ribs, as in gatun- 



Jahrb. k.k. geol. Reichs., Bd. 61, p. 493, pi. 30, fig. 4, 1911. 



