120 STUDIES IN GEOLOGY, No. 3 



like furrows which are sometimes more or less irregular, and 

 which are crossed by longitudinal imbricated channels near 

 the hinge. Teeth in a single series, fine, rather long, 

 slightly oblique and longer distally. Inner margin fluted. 

 Length, 40; height, 40.5; (semi) diameter, 18.5; length of 

 hinge, 30 mm. 



This species is very closely related to others in the Zorritos 

 fauna, and might be termed a mutant or variant of some of 

 them if the special characters which serve to distinguish it 

 were less constant or if there were more perceptible inter- 

 grading. Area toroensis is distinguished from it by its larger 

 and more irregularly furrowed area, its generally less pro- 

 nounced marginal furrow and band bounding the area, and 

 ijts tendency to be angular and to flare ventrally; A. nelsoni 

 is always smoothly rounded. A. nelsoni is distinct from A. 

 septifera in its smaller and less furrowed area and its gen- 

 erally finer teeth. In shape and other characters the two 

 agree. A. septifera seems to attain a larger size than nelsoni, 

 and its -ribs are ordinarily coarser, but on the whole the two 

 are very similar, and are separable chiefly on account of 

 thfe difference in area. A. nelsoni differs from the other 

 related Areas of the local fauna proportionally as they differ 

 from the forms noted; it is somewhat like the A. imporcata 

 group, but differs more clearly from them than from the 

 forms mentioned. 



Outside of Peru its closest relatives are to be found in the 

 Caribbean Oligocene and Miocene. Scapharca anguillana 

 Cooke 12 from the Oligocene of Antigua, is very close, but 

 has more prominent tuberculation of the ribs, the area bears 

 eight furrows, the teeth are shorter, and the shell flares 

 ventrally; that of nelsoni does not. It differs less promin- 

 ently from. A. chiriquiensis Gabb (2) from the Miocene of 



120 Carnegie Inst. Wash., Publ. 291, p. 127, pi. 5, figs. loa, b, 1919. 

 (2) Maury, Bull. Amer. Pal., vol. 5, No. 29, p. 174, pi. 28, fig. 

 2, 1917- 



