io2 STUDIES IN GEOLOGY, No. 3 



The Hopkins collection contains one complete specimen 

 of this form, and although it is coated with calcium carbon- 

 ate, with the definition of some of its characters thus some- 

 what lost, its difference from other similar forms makes 

 clear its identity as a distinct species, and warrants descrip- 

 tion. It is very close to A. zorritensis, but differs in the 

 number of ribs and in being much less oblique, the ventral 

 margin being much more nearly parallel with the hinge. It 

 is also somewhat similar to A. pantheonensis, of the Varie- 

 gated fauna, but is less inflated, smaller, and has not so 

 wide an area. 



The group of A. singewaldi, of the Zorritos fauna, A. 

 proletaria f Br. & Pils. and A. intunmlata Br. & Pils. 96 of the 

 Santo Domingan Oligocene, and A. improcera Conrad, 97 of 

 the Miocene of North Carolina, is similar, differing as a 

 whole mainly in the greater number of ribs and the narrower 

 interspaces. Anadara darwini (Mayer), described by 

 Sacco 98 from the Tortonian to the Astian of the Italian 

 Piedmont, is similar in shape, but is more inflated, and has 

 broader ribs. A. donacia Dall," from the lower Miocene 

 of Jamaica, is somewhat similar, but is much smaller, has 

 comparatively lower beaks, is longer in proportion to height, 

 has less ribs, and lacks the nodulose rib-sculpture. 



Lower Zorritos. Quebrada Zapotal. 



Area (Scapharca) fissicosta n. sp. 

 PLATE V Fig. .11. 



Shell of medium size, inflated, smoothly cpnvex. Ante- 

 rior margin broadly rounded, somewhat produced ; ventral 

 margin elliptical, in the main parallel to the hinge-line ; pos- 

 terior margin slightly curved, meeting hinge at an angle of 



"Proc. Ac. Nat. Sc. Phila., vol. 69, p. 188, 1917. 



9T Foss. Med. Tort, p. 60, pi. 31, fig. 5, 1845. 



"Piemonte, pte. 26, p. 24, pi. 5, figs. 11, 12, 1898. 



"Trans. Wag. Inst, vol. 3, pt. 4, p. 649, pi. 33, fig. 13, 1898. 



