528 MEMOIRS OP THE CARNEGIE MUSEUM. 



a 3'Oiing ellipticus, and may correspond to the var. santnnus of Von Ihoring (smaller, 

 projection of lower margin less distinct). 



23. DiPLODON BERTHS Ortmann, sp. nov. 

 Shells: Plate XXXVIII, figs. 1, 2, 3, 4; Anatoimj of gills: Plate XLVI, fig. 6. 



Type-locality. — Rio Jacuhy, Cachoeira, Rio Grande do Sul, Brazil (J. D. Hase- 

 man coll., January 26, 1909). Tijpe-sei: Carn. Mus. Cat. No. 61.5865. Sixteen 

 specimens, all with soft parts, including males, barren and gravid females. (There 

 were twenty-three specimens in the original lot). 



Additional Locality. — Rio Vaccahy Mirim, Santa Maria, Rio Grande do Sul, 

 Brazil (J. D. Haseman coll., January 29. 1909). One male with soft parts. 



Distribution: Guahyba drainage in southern Brazil. 



Description of Shell. — Shell rather small, maximum length 65 mm.; rather 

 solid, chiefly so anteriorly, and in old shells often much thickened along the lower 

 anterior margin. Outline subovate to subtrapezoidal, distinctly oblique, broad 

 and rounded, or somewhat pointed behind. Height from 55 to 67 pr. ct. of length. 

 Valves not gaping. Dorsal margin straight, or gently descending posteriorly, 

 forming a more or less distinct obtuse angle with the posterior margin. The latter 

 obliquely descending, gently convex, and curving around into the posterior part 

 of the lower margin, forming with the latter in young specimens an indistinct 

 rounded angle, which, however, may become more distinct in old specimens. Lower 

 margin in normal specimens with a distinct rounded projection, forming the lowest 

 point of this margin, situated far back, behind the posterior end of the ligament. 

 From this point the lower margin curves up behind to the posterior end of the shell 

 and this part is quite short. Anteriorly the lower margin also slopes upward, 

 and is almost straight for a considerable distance; sometimes it is even slighth^ 

 concave; then it curves up into the anterior margin. Thus the shell appears con- 

 siderably narrower anteriorly, broader (higher) posteriorly, with the greatest 

 height situated far backward. 



Valves moderately and not uniformly convex. The greatest convexity is near 

 the anterior end and over the posterior ridge, which is broad and not sharply marked. 

 In front of the posterior ridge the sides of the disk are distinctly and broadly flat- 

 tened, and sometimes even slightly concave, producing the emargination of the 

 anterior part of the lower margin. Posterior slope somewhat compressed, very 

 rarely with a slight trace of a rib or a furrow. Greatest diameter of the shell 32 

 to 43 pr. ct. of the length, located well behind, upon, or close in front of, the pos- 

 terior ridge. Thus, although rather swollen in the region of the jiosterior ridge, 



