98 STUDIES IN GEOLOGY, No. 3 



is very close in shape to A. alcima Ball 89 from the Caloosa- 

 hatchie Pliocene, but its area is not wide, as is that of alcima, 

 the number of ribs is 25 instead of 30, and there is no marked 

 discrepancy in the two valves either in this respect or in the 

 matter of size; the anterior-ventral margin descends at a 

 higher angle in zorritensis, and the nodulation of the ribs 

 is not as sharp. The beaks of alcima are a bit higher. 



A. zorritensis has most of the characteristics of a Cunearca, 

 but is very near the border line between that section and 

 Scapharca s.s., if, indeed, such a line may be said to exist. 

 It has the area, discrepant sculpture, and general shape of a 

 Citnearca, and although the discrepancy on the valves is no 

 greater than that of the average Scapharca s.s., it seems 

 advisable to place it in the section C linear c a. Area filicata 

 Guppy, 90 from the lower Oligocene of Trinidad, is the most 

 closely related form. It differs in having 30 ribs instead of 

 23, the ribs being broader than the interspaces, instead of 

 narrower as in sorritensis, and in being less produced ante- 

 riorly. The sculpture of the ribs is the same, and the general 

 shape and size is very little different. In the local fauna A. 

 pantheonensis, from the Variegated Beds, is very similar, 

 but differs in being larger, having a wider area, and in its 

 more pronounced and opposite discrepancy of the valves. 



Zorritos formation. Zorritos. 



Area (Cunearca) sp. indet. 

 PLATE V Figs. 6, 7. 



Shell high, inflated. Outline unknown. Umbones high, 

 erect, not compressed. Umbonal slope rounded in contour 

 anteriorly and mesially ; a distinct posterior keel curves out- 

 ward toward the margin, and the surface behind it is 

 excavated. Ribs, 29 in number, decorated with many non- 



89 Trans. Wag. Inst, vol. 3, pt. 4, p. 635, pi. 31, figs. 5, 7, 1898. 

 90 Quart. Jour. Geol. Soc. Lond., vol. 22, p. 583 pi 26 fig 5 

 -866. 



