Paleontology Zorritos Formation 99 



concentrically arranged, cog-shaped nodes. On the ventral 

 parts of the extreme posterior ribs the nodes flatten to insig- 

 nificance. Area broad, short, transversely striate, bounded 

 bv a distinct furrow. Teeth (few visible) fine, of medium 

 length. 



This form is very close to A. alcinia Dall, and the group 

 of related forms. It is very similar to A. chemnitzioides 

 Maury ,®^ from the Oligocene (or later) of Trinidad, but it 

 has not the concentrically arranged nodulation of that form, 

 and has a much more pronounced posterior keel. The area 

 is not as high as that figured by Maury, and is delimited by 

 a marginal furrow not evident in the figure of chemnltiz- 

 ioides; the point of the beak is fuller and more incurved, and 

 the teeth are apparently not as long. It diflfers from A. 

 alcima Dall ^'- in having finer, rounder, and less regular 

 nodulation of the ribs. The posterior keel is curved out- 

 ward ; that of alcima is nearly straight. Otherwise the char- 

 acters of the upper part of the left valve agree with those 

 of alcima. Although the visible characters of this fragment 

 show that it is probably a new species, naming of it is 

 deferred until a full description can be made. 



Variegated. Quebrada Tusillal. 



Section Scapharca s.s. 



Area (Scapharca) panihconcnsis n. sp. 



Plate V — Figs. 8, 9. 



Shell of medium size, inequivalve, the right valve higher, 

 moderately inflated. Anterior margin slightly rounded, for 

 the most part at right angles to the hinge, but curving in 

 slightly to meet the hinge-line. \'entral margin rounded, its 

 main line parallel to the hinge ; posterior margin straight 

 centrally, curving sharply into the ventral margin and curv- 



"Jour. Ac. Nat. Sc. Phila., ser. i. vol. 15, p. 44, pi. 7, figs. 13, 

 14. 15. pi. 8, fig. I, 1912- 

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



