DIPLODON 1249 



dermis olive-green or brownish, somewhat shining, often show- 

 ing" the rest marks ; left valve with one compressed pseudocar- 

 dinal, sometimes with a small one in front, the lower surface 

 of the tooth often excavated, and two slender laterals ; right 

 valve with two pseudocardinals, the lower larger and ragged 

 and one lateral ; dorsal scars small and numerous ; anterior 

 scars impressed ; there is a small deep crescent-shaped scar at 

 the base of the pseudocardinals ; ivicre bluish-white, often 

 blotched, a little thickened in front. 



Length 73, height 42, diam. 24 mm. 



Lake Portero near Alaldonado, Uruguay. 



I feel vei-y great hesitation about launching new species of 

 the genus Diplodon, but there are several forms from Southern 

 Brazil and adjacent regions that I cannot refer to any known 

 species and the above is one of them. It has no striking char- 

 acters, but is a simple, elongated, scarcely inflated form, often 

 slightly truncated anteriorly. Some specimens show feeble, 

 radial sculpture on the center of the disk, and it sometimes be- 

 comes plicate. Other shells are entirely destitute of such 

 sculpture. 



D1P1.ODON MiMi;.^ n. s. 



Shell small, long rhomboid, convex, solid, inequilateral, the 

 beaks much eroded in the specimens seen ; posterior ridge low, 

 widely and decidedly double, ending behind in a wide biangula- 

 tion at the base line ; dorsal and ventral lines nearly parallel ; 

 surface very finely, concentrically sculptured ; epidennis brown, 

 tawny on the center of the disk, scarcely shining; left valve 

 with two pseudocardinals, the posterior small, and two remote 

 laterals ; right valve with two pseudocardinals, the lower 

 strong, . and one lateral ; muscle scars well impressed ; beak 

 cavities shallow, with a few pits ; nacre dull whitish, lurid pur- 

 ple within, thicker in front. 



Length 7)7' height 28, diam. 17 mm. 



Iguape, Brazil. 



A small solid, rhomboid species, which almost exactly mim- 

 ics in external appearance some of the varieties of Unio com- 

 planatiis. The dirty nacre is finely, radially sculptured out- 

 side the pallial line in some specimens. 



