PALEONTOLOGY ZORRITOS FORMATION 113 



in the fauna that some mention of it is pertinent. It is 

 another of the forms on the border-line between the Cunear- 

 cas and the true Scapharcas, "being in this respect much like 

 A. pantheonensis except for the equality of its valves. It has 

 the smooth area set off by a marked groove, the character- 

 istic teeth, and the general appearance o>f a Cunearca, but its 

 valves are nearly equal in size, and the sculpture is not dis- 

 crepant. A. pantheonensis has the discrepant valves, but 

 has a distinct furrow on the area. 



Its nearest relatives are in the Zorritos fauna. Area pan- 

 theonensis bears the relationship noted above, but in addi- 

 tion to those differences it is more oblique in shape, and has 

 more regularly nodulate sculpture instead of the irregularly 

 tuberculate ribs of A. larkinii. A. imporcata is very similar 

 to A. larkinii, being almost indistinguishable from it in 

 general appearance, but it has consistently less ribs, and the 

 area is deeply furrowed; that of larkinii is smooth. A. 

 tuberculosa Sowerby, 114 of the recent Peruvian fauna, likened 

 by Nelson to his A. larkinii is similar only in the tuberculose 

 character of the ribs, being much more oblique, inequilateral, 

 and produced than larkinii. The recent peruvian A. grandis 

 Brod. & Sow. 115 is much larger, but is similar in shape. 

 Both tuberculosa and grandis are clearly true scapharcas. 



Area (Scapharca) imporcata n. sp. 

 PLATE V Figs. 19, 20. 



Area larkinii Grzybowski, Neues Jahrb. f. Min., etc., Beil. Bd. 

 12, p. 633, 1899, (not of Nelson, 1870). 



Shell thick, medium-sized, sub-equilateral, rhomboidal. 

 Anterior margin straight at juncture with hinge, curving 

 gently into ventral margin, which is normally straight and 

 nearly parallel to the hinge. Posterior margin little if at all 



ll4 Dall, W. H., Proc. U. S. Nat. Mus, vol. 37, p. 154, pi. 27, 

 fig. 4, 1910. 



115 Idem., pi. 25, figs. 9, 10. 



