96 Studies in Geology, No. 3 



striations of the ligament area before the beaks. Length 

 (left valve) 32; height, 24.5; (semi) diam., 12 mm. Length 

 (right valve) 35; height, 28; (semi) diam., 12.5 mm. Length 

 ligament area, left valve, 11 ; right valve, 12 mm. 



This species is similar to A. modesta Grzybowski,^*' from 

 Zorritos, but dififers in shape, being decidedly lower and 

 more squat in appearance. Other points of similarity or 

 difference may exist, but are not notable here on account of 

 the meagerness of Grzybowski's description and the lack of 

 detail in his figures. Comparison with his figure reveals a 

 broad difference in shape. . A. cholana is lower and broader 

 than A. trinitaria Guppy,^'^ and the posterior keel is not pro- 

 duced downward as in trinitaria. The beaks of trinitaria are 

 fuller, higher, rounder viewed posteriorly, and more acute 

 viewed laterally. The anterior margin of trinitaria is less 

 produced, and the posterior surface less excavated and less 

 alate. A. sheldoniana Maury ,^® from the upper Oligocene 

 (Aquitanien) of Trinidad, is similar in shape and ribbing, 

 but is much smaller. The area seems to be different, but 

 Maury's description is inadequate, and the plates do not show 

 sufficient detail for fruitful comparison. 



Variegated (near base). Quebrada Tusillal. 



Subgenus Scapharca (Gray) Dall 



Section Cunearca Dall 



Area (Cnnearca) zorritensis n. sp. 



Plate V — Figs. 4, 5. 



Scapharca sp. ind. Nelson, Trans. Conn. Acad., vol. 2, p. 205, 

 1870. 



"Neues Jahrb., Bcil. Bd. 12, p. 635, pi. 28, figs. 4, 4a, 1899. 



"Quart. Jour. Geo!. Soc. Lond., vol. 22, p. 583, pi. 26, figs. 3a, 

 3b, 1866. 



'*Jour. Ac. Nat. Sci. Phila., 2nd ser. vol. 15, p. 43, pi. 8, figs. 

 ID, II, 1912. 



