Paleontology Zorritos Formation hi 



of A. santarosana Dall. Xoclules tend to become absent 

 near the ventral margin about the umbonal keel, and the 

 ribs show a shallow longitudinal sulcus for a short distance 

 above the ventral margin, the sulcus being more prominent 

 mesially. Area short, narrow, with two furrows. Teeth 

 small, with comparatively broad interspaces, the hinge-plate 

 becoming wider posteriorly. Length, i8; height, 20; (semi) 

 diameter, 10 mm. 



Only one imperfect left valve of this form is present in the 

 Hopkins collection. It offers a difference from Maury's 

 figured A. Iiispaniolana in that its ribs are uniformly equal 

 width to the interspaces, whereas the ribs of hispaniolana are 

 broader than the interspaces and there is further difference 

 in the ventrally sulcate ribs of this form, Maury's figure 

 and description giving no hint of such structure, but the 

 presence of the one imperfect specimen only makes differ- 

 entiation of the form unwise, and unless further specimens 

 of the Peruvian form show it to have constant differences it 

 may well be united with hispaniolana. 



The sculpture and general appearance of the shell suggest 

 strongly A. (Cunearca) alcima Dall,^^^ but it is clearly a 

 typical Scapharca, and in addition is much smaller, with 20 

 instead of 30 ribs. Area chemnitzii Philippi is also similar 

 in appearance, but it, as well as alcima, is a Cunearca. A. 

 chemnitzio-ides Maury.^^^ from the Oligocene of Trinidad, 

 is similar, but in addition to being a Cunearca it has a greater 

 number of ribs, and is posteriorly alate instead of truncate, 

 as is hispaniolana. 



Lower Zorritos. Quebrada Zapotal. 



Area (Scapharca.^) larkinii Xelson 



Plate V — Figs. 16-18. 



Area Larkinii Xelson, Trans. Conn. Acad., vol. 2, p. 204, pi. 7, 

 figs. 5, 6, 7, 1870, (not Area Larkinii Grzybowski 1899) 



Trans. Wag. Inst., vol. 3, pt. 4. p. 635, pl- 3^, figs. 5, 7, 1898. 

 'Jour. Ac. Nat. Sc. Phila., vol. 15, p. 44, pl. 7, fig. 15, 1912. 



