DIPI.ODON I 23 1 



blunt point behind about at the base of the shell ; above it on 

 the posterior slope there is a wide, shallow, radial depression ; 

 surface with line, concentric growth lines ; epidermis chestnut- 

 color, shaded with green at the umbonal region, smooth and 

 shining except on the somewhat roughened border ; left valve 

 with two radial pseudocardinals, which are much split up and 

 one granular lateral, with a vestige of a second above it ; 

 right valve with two pseudocardinals, the lower greatly di- 

 vided, the upper compressed : behind these there is a vestigial 

 tooth, and one somewhat double, granular lateral ; dorsal scars 

 large ; anterior scars deep ; posterior scars well-impressed ; 

 nacre white, almost silvery, thickened in front and iridescent 

 behind. 



Length TOi, height 66. diam. 40 mm. 



Uruguay River ; South America. 

 Uriio nrymanii Lha, Pr. Ac. N. Sci. Phila., IV, i860, p. 90; 



Jl. Ac. N. Sci. Phila., VI, 1863, p. 381, pi. xui, fig. 280; 



Obs., X. 1863. p. \y, pi. xTji. fig. 289. — SowERBY, Conch. 



Icon., XVI, t868, pi. lxxxiv, fig. 449. 

 Margaron (Unio) wymanii Lka, Syn., 1870, p. 35. 

 Diplodon ivymanii Simpson, Syn., 1900, p. 874. 

 Unio apprimus Lea, Pr. Ac. N. Sci. Phila., X, 1866, p. 34 : Jl. 



Ac. N. Sci. Phila., VI, 1868, p. 263, pi. xxxiii, fig. 78 ; Obs., 



XII, 1869, p. 22,, pi. xxxiii, fig. 78. 

 Margaron Unio apprimus Lea. Syn., 1870, p. 46. 

 Diplodon apprimus Simpsox, Syn., 1900, p. 874. 



This is perhaps the finest Diplodon known. It is a solid, 

 rhomboid shell with a fine, shining chestnut epidermis, the sur- 

 face delicately concentricall}- sculptured above, the sculpture 

 becoming more rude on the border. The pseudocardinals are 

 much split up ; the nacre is white and fine. I am now con- 

 vinced that Lea's U. wymani and apprimus are identical. 



DiPEODON EXPANSUS (Kuster). 



"Shell subovate, convex, solid, irregularly striate, shining, 

 reddish-brown, rounded in front, subtruncate behind ; um- 

 bones flat, eroded ; dorsal slope compressed, obtusely angulat- 



