1272 DIPLODON 



Almost evenly elliptical in outline. There is sometimes a 

 very slight post-dorsal wing and the base line is occasional h 

 a little produced behind the middle. Old shells become quite 

 dark and the banding fades out ; in younger specimens the 

 bands are quite distinct. 



DiPi^ODON TRiFiDiTS (Lea), 



Shell long ovate, solid, scarcely inflated, very inequilateral, 

 with high, rather full beaks, which are sculptured with numer- 

 ous, strong, radial bars that extend well out on to the disk ; 

 posterior ridge strong and decidedly angled, ending behind in 

 a point a little below the median line; surface with faint, con- 

 centric sculpture ; epidermis smooth, almost shining, dark and 

 light olive-green in concentric bands ; left valve with two high, 

 very ragged pseudocardinals with a feeble third one between 

 them, the middle one split up, and two curved laterals ; right 

 valve with one strong, ragged pseudocardinal, a very small 

 one above it, and a divided lateral ; muscle scars small, im- 

 pressed ; dorsal scars distinct ; nacre bluish-white, somewhat 

 silvery. 



Length 42, height 22, diam. 15 mm. 



Argentina. 

 Unio trindus Lka, Pr. Ac. N. Sci. Phila.. IV. i860, p. 89: Jl. 



Ac. N. Sci. Phila., V. 1863, p. 386, pi. xuv, fig. 295 ; Obs., 



X, 1863, p. 22, pi. xuv, fig. 295. 

 Margaron (Unio) triUdus Lka, Syn., 1870, p. 50. 

 Diplodon trifidits Simpson, Syn., 1900, p. 884. 



The type, the only specimen 1 have seen, may be a young 

 shell, though it is quite solid. This species is more solid and 

 elongated than D. burromgJiimius and its pseudocardinals are 

 much ruder and rougher. 



DiPLODOx iiiERixc.T (Clessin?). 



Shell small, nearl}- evenly elliptical, inequilateral, subsolid, 

 convex, with a well developed, subangular posterior ridge, 

 which ends behind in a point at the median line ; beaks mod- 

 erateh' full, not much elevated above the dorsal line, with 



