Paleontology Zorritos Formation 105 



angle of 120° instead of 140,° as in singez>.'aldi, and the angle 

 between the posterior and ventral margins is accordingly less 

 acute; the umbones are similar except that the sulcus in 

 prolctaria is slightly less perceptible, and does not extend 

 as far from the point of the beaks ; the surface of the valves 

 is identical for the two forms anterior-mesially, but the pos- 

 terior keel is slightly less sharp in proletaria, and the sur- 

 face adjacent to the posterior margin is excavated, the mar- 

 ginal surface flattening out somewhat, whereas in singeii'aldi 

 there is no such flattening, and the valves have a much more 

 solid appearance. The ribs of proletaria are distinctly 

 broader than the interspaces, and do not show the marginal 

 sulcation characteristic of singeii'aldi; the area of proletaria 

 is proportionally longer and much more narrow' ; the teeth 

 are similar, but there is no break in the series of singezvaldi, 

 and the hinge of proletaria is somewhat longer. In general 

 contour the shells are decidedly similar. 



Area intumulata is longer than singewaldi; otherwise it 

 differs much as does proletaria, but has the following points 

 of similarity not present in that species : some of the ribs of 

 intumulata are sulcate very nearly as distinctly as those of 

 singewaldi; the area, though longer, is similar in appear- 

 ance ; the sulcation of the beak is similar ; and the teeth are 

 in a continuous row, though coarser. The nodes on the ribs 

 of intumulata are not as prominent as those of singeii'aldi. 



A. singezvaJdi is also strikingly similar in general appear- 

 ance to A. improcera Conrad,^°^ and the related forms, A. 

 improcera var. buccula and A. plicatura Conrad.^"* Compar- 

 ison with specimens of these forms from the Miocene of 

 North Carolina shows improcera to be less inflated, longer, 

 with proportionally wider ribs which are plain instead of 

 nodulate, and with the valves meeting in an angle more acute 

 than that of singewaldi ; plicatura is slightly less inflated, the 



Foss. Med. Tert., p. 60. pi. 31, figs. 5, 18, 1852. 

 Idem., p. 61, pi. 32, fig. 4. 



