1284 DIPI,ODON 



compressed, solid, inequilateral ; dorsal line almost regularly 

 curved from the anterior to the posterior ends, very high in 

 the middle where it is slightly winged ; beaks rather low, not 

 elevated above the dorsal line ; posterior ridge low ; above it is 

 a shallow, wide, radial depression, which ends in a feeble sinus 

 behind ; base line rounded, lull behind the middle ; surface 

 with low, fine, concentric ridges, with a trace of radial sculp- 

 ture ; epidemiis almost black, thick, scarcely shining, peeling 

 ofif and showing the white shell beneath ; left valve with one 

 much-split-up pseudocardinal and two short, remote, strongly 

 curved laterals ; right valve with two pseudocardinals, the 

 upper compressed and small, the lower dentilate, with a ves- 

 tigial tooth behind it, and one curved lateral ; muscle scars 

 small ; nacre white, dull, blotched in the shallow cavity, very 

 thick along the anterior border where it is granularly, radially 

 striate ; pallial line remote from the border. 



Length y^, height 67, diam. 23 mm. 



Uruguay River. 

 JJnio fttnebralis Lka, Pr. Ac. N. Sci. Phila., l\\ i860, p. 91 ; 



Jl. Ac. N. Sci. Phila., V, 1863, p. 378, pi. xlt, fig. 286; Obs., 



X, 1863, p. 14, pi. xu, fig. 286. — SowERBY, Conch. Icon., 



XVI, 1867, pi. LviT, fig. 290. 

 Margaron CUnio) fnnebralis Lka, S}"n., 1870, p. 55. 

 Diplodon fnnchralis Simpson, Syn., 1900, p. 887. 



The decided compression, the very high, curved dorsal line, 

 the thickened nacre in front, which is granularly, radially 

 striate, and the pallial line set far in from the border distin- 

 guish this species from others. 



DiPi.ODON PARANENSis (Lea). 



Shell disposed to be quadrate or pentagonal, subinflated to 

 inflated, solid, inequilateral ; beaks high and somewhat full, 

 sculptured with strong, uneven, radial ribs ; area in front of 

 them incurved, area behind them curved outward ; posterior 

 slope truncated, the outline sometimes incurved ; posterior end 

 blunt ; base very much swollen at or a little behind the middle ; 

 anterior end narrow and roimded ; surface covered with coarse, 

 uneven, concentric sculpture; epidermis brownish to blackish: 



