no STUDIES IN GKOLOGY, No. 3 



are about equal in width to the interspaces, sometimes nar- 

 rower. The valves 'are often slightly ^discrepant in height, 

 the right valve being higher. 



This species differs from A. imporcata chiefly in its more 

 produced posterior margin, more inclined beaks, and conse- 

 quent more oblique shape. Its valves are sometimes slightly 

 discrepant in height, but never prominently so, as are those 

 of Area crescens, and it is further distinguishable from 

 crescens by its less oblique outline. Grzybowski notes the 

 similarity between obesiformis and A. cardiiformis Bast., 117 

 from the Miocene of Europe. That form is less produced, 

 and has no pronounced posterior keel; the furrows of its 

 area are more regular and more nearly straight and longitu- 

 dinal, meeting the marginal furrow at angles, whereas the 

 furrows in the species under discussion are more irregular, 

 and either branch into the marginal furrow or meet at the 

 hinge-line. A. obesiformis is even more closely similar to 

 A. idonea Conrad 118 than is A. imporcata. It differs, how- 

 ever, in having a much broader area. 



Upper Zorritos. Quebrada del Toro. 



Area (Scapharca) crescens n, sp. 

 PLATE VI Figs. 3, 4. 



Shell of medium size, heavy, oblique, the right valve 

 higher than the left. Anterior margin bluntly rounded, 

 meeting the hinge at an angle, though obtuse, very nearly 

 90. Ventral margin long, slightly bowed, nearly parallel 

 to the hinge-line in its posterior half; posterior margin 

 greatly produced, forming an acute angle with the ventral 

 margin, and, beyond the curved apex of this angle, nearly a 

 straight line, forming an angle of 150 155 with the 

 hinge-line. Umbones prominent, inflated, prosogyrate, and 



11T Hoernes, M., Abh. k.k. geol. Reichs., Bd. 4, p. 331, pi. 43, 

 figs- 3, 4, 5, 1870. 

 118 Md. Geol. Surv., Miocene, p. 389, pi. 106, figs. I, 2, 1904. 



