io6 STUDIES IN GEOLOGY, No. 3 



posterior surface is more flattened, and not as full, the 

 beaks are more dome-like, and the valves are less oblique. 

 The resemblance to var. buccula is stronger, possibly, than 

 to the others in shape, degree of inflation, size, shape and 

 number of ribs, but buccula is more produced anteriorly, 

 the anterior margin being more fully rounded, and is less 

 produced posteriorly ; the ribs are little or not at all nodulose, 

 and the distal teeth are coarser and longer. In some speci- 

 mens of buccula the beaks are nearly as high as those of 

 singewaldi, but in such the hinge-line is shorter. 



A. singewaldi differs from A. transversa Say 105 in being 

 more inflated, thus more rounded in general appearance, 

 more produced and flatter anteriorly, with lower, less in- 

 flated beaks, and smaller area. 



Lower Zorritos. Quebrada Zapotal and quebrada north 

 of Quebrada Boca Pan. 



Area (Scapharca} singewaldi var. doma n. var. 



Characters agree with those of A. singewaldi except as 

 follows : This form has 50 ribs instead of 33, with the pos- 

 terior ribs flatter than those of singewaldi. The nodulation 

 of the ribs is finer and less distinct. The posterior slope of 

 the shell is more blunt than that of A. singewaldi, the angle 

 between the posterior margin and the hinge being nearer to 

 125 than 140, and the posterior margin curving back, 

 lessening the production of the shell posteriorly. The um- 

 bones are not sulcate. The area is narrower. Length, 19; 

 height, 17; (semi) diam., 8.5 mm. Length of hinge, 13.75 

 mm. 



Lower Zorritos. Quebrada Zapotal. 



Area (Scapharca) vanhols.fi n. sp. 



PLATE: V Fig. 14. 



Shell small, thin, only moderately inflated, of slightly 

 103 Idem., p. 15, pi. i, fig. 2 . 



