Paleontology Zorritos Formation 117 



inclined forward. Umbonal slope rounded anteriorly and 

 mesially, with a distinct posterior keel sloping backward at 

 a low angle. Ribs 2"] to 28 in number, pronounced, rounded, 

 with tubercles anteriorly, plain posteriorly, tending to become 

 lost on the nearly flat posterior surface of the valves. Ribs 

 narrow, the interspaces often wider, but usually about equal. 

 Area long and broad, with the furrows and striations char- 

 acteristic of this group. Teeth fine and short, increasing in 

 size and coarseness distally. Inner margin fluted. Length. 

 47.5 mm ; height of right valve, 37.5 mm. ; of left valve, 

 32.5 mm. ; diameter, 39.75 mm. 



This species is distinguishable from A. imporcata and A. 

 obesiformis, its nearest relatives, chiefly through its shape; 

 it is much more produced, long, and oblique. The discrep- 

 ancy in height of the valves affords a clear distinction from 

 imporcata, whose valves are equal in height, and from obesi- 

 formis, in some forms of which there is a slight discrepancy, 

 but never a marked one. The ribs are narrower and more 

 widely spaced than in either of the other two forms. The 

 umbones are usually much more distant. 



Upper Zorritos. Mouth of Quebrada del Grillo. 



Section Axadara Grav 



Area (Anadara) scptifera Grzybowski 



Plate VI— Figs. 5, 6. 



Area scptifera Grzybowski, Xeues Jahrb., Beil. Bd. 12, p. 633, 

 pi. 13, fig. 2, 2a, 1899. 



Shell large, heavy, very thick (thickness of 9.5 mm. meas- 

 ured on broken left valve) . rounded in contour. Anterior mar- 

 gin broadly rounded and somewhat produced, forming a right 

 angle at its juncture with the hinge-line; ventral margin 

 rounded as far as known ; posterior margin broken awav in 

 all specimens. Umbones high, incurved, nearly erect, upper 

 surface about 18 mm. vertically above the hinge line in the 

 average specimen. Umbones about 10 mm. distant. Um- 



