DIPLODON 1283 



Margaron fUnio) rotundus h^\, Syn., 1870, p. 56. 

 Diplodon rotundus Simpson, Syn., 1900, p. 886. 



The shell figured in the Testacea Fluviatilia is apparently 

 quite young and, it the figure is correct, it is faintly rayed. I 

 have before me several shells which belong to the Lea Collec- 

 tion, which were believed by Dr. Lea to be the Unio rotundus 

 of Wagner, and I am inclined to think that they are. The sur- 

 face is rather smooth and shining and has a peculiar soft 

 smoky tint. The nacre is rich and bright for a Diplodon. 



Diplodon disculus (Lea). 



Shell much compressed, subsolid, short obovate, inequilat- 

 eral, with moderately full beaks, whose sculpture is irregularly 

 radial ; dorsal line incurved in front of the beaks and elevated 

 behind them; post-dorsal region elevated into a slight wing, 

 which is abruptly and almost squarely truncated posteriorly ; 

 posterior ridge rounded ; anterior end narrowed and rounded ; 

 posterior base full; surface finely, concentrically striate; epi- 

 dermis dirty green, sometimes faintly rayed ; left valve with 

 one split-up pseudocardinal and two curved laterals ; right 

 valve with two pseudocardinals, the lower dentilate or divid- 

 ed, and one lateral ; nacre bluish, thicker in front. 



Length 40, height 36, diam. 15 mm. 



Length 45, height 40, diam. 15 mm. 



Uruguay River, South America. 

 Unio discuhis Lea, Pr. Ac. N. Sci. Phila., IV, i860, p. 91 ; Jl. 



Ac. N. Sci. Phila., V, 1863, P- 385, pi- xuv, fig. 293 ; Obs., 



X, 1863, p. 21, pi. XLiv, fig. 293. 

 Margaron (Unio) disculus Lea, Syn., 1870, p. 55.. 

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



All the material, which I have seen, including the type, is 

 probably young. The shell is much compressed, narrowed and 

 rounded in front, largely produced at the post-base and widely, 

 almost squarely truncated behind. It is much more squarely 

 truncate behind than any closely related species. 



Diplodon r"UNEBRALis (Lea). 



Shell nearly orbicular, but a little narrowed in front, being 

 slightly drawn out and rounded at the upper anterior part. 



