666 PROCEEDINGS OF THE ACADEMY OF [DeC, 



behind, producing a subangulation above and below, scarcely alate; 

 beaks rather low, sculptured with strong ribs, nearly vertical in front, 

 more radiating posteriorly and changing abruptly to nearly horizontal 

 in the posterior sulcus; everywhere marked by strong concentric 

 growth lines which decussate the beak sculpture and become almost 

 lamellate in the posterior sulcus, and very faintly marked by obsolete 

 radiating lines discoverable over most of the shell; posterior ridge 

 low, posterior gToove shallow and flat ; hinge line much curved, right 

 valve with a large and a small compressed pseudocardinal, which 

 are nearly parallel, the upper (anterior) thin, straight, and regular, 

 the other much heavier, but varying in different specimens, both 

 variously corrugated and lined on their adjacent surfaces and nearly 

 smooth on their distal surfaces, and, on the same valve, a long, slender 

 lateral which is generally roughened on the inner surface and minutely, 

 irregularly serrate on the edge ; left valve with two compressed pseu- 

 docardinals, one in front of the other, the anterior one roughened 

 and much the larger, and two long, slender, and nearly parallel 

 laterals ; beak cavities moderately deep ; cicatrices deeply impressed 

 and well defined, the anterior irregularly incised on the posterior 

 edge; nacre not brilliant, bluish -white, slightly iridescent behind. 



Length 59.5, width 37.5, diam. 28 mm. 



Thirteen specimens of this species, which seems to be new, taken 

 in the Rio Jamauchim, were sent from the Goeldi Museum. The 

 species is named for Professor F. W. Kelsey, of San Diego, Cal., 

 who has kindly undertaken the photographing of the shells illus- 

 trated in this paper. 

 Diplodon obsolescens n. sp. PI. XXII, figs. 16, 17. 



Shell rather thin, elongate subrhomboidal, rather full, very inequi- 

 partite; beaks low, eroded, but apparently presenting the peculiar 

 radial sculpture characteristic of the genus; everywhere marked by 

 concentric growth lines and with about two-fifths of the disk marked 

 by extensions of the radial sculpture of the beaks; epidermis rather 

 heavy, black, rayless; anterior margin rather evenly rounded; basal 

 margin nearly straight; posterior margin truncate with evenly 

 rounded upper and lower angles; superior margin slightly curved; 

 hinge line distinctly angulate below the umbones; teeth very poorly 

 developed, the outer (anterior) pseudocardinal of the right valve 

 being restricted to a narrow, scarcely perceptible ridge, the other 

 being broader but only slightly more elevated, the long lateral a 

 little more prominent than either pseudocardinal; pseudocardinals 

 of the left valve equally obsolete, while the two laterals are fairly 



