574 MEMOIRS OF THE CARNEGIE MUSEUM. 



New Localilies. — Rio Uruguay (in mud), Uruguayana, Rio Grande do Sul, 

 Brazil (J. D. Haseman coll., February 5, 1909). Four specimens with soft parts, 

 males and females. Rio Cacequy, Cacequy, Rio Grande do Sul, Brazil (tributary 

 to Rio Ibicuhy, to Uruguay) (J. D. Haseman coll., February 2, 1909). Two speci- 

 mens with soft parts, male and female. Rio Jacuhy, Cachoeira, Rio Grande do 

 Sul, Brazil (tributary to Rio Guahyba) (J. D. Haseman coll., January 26, 1909). 

 Two specimens. 



Distribution. — Known from the Rio de la Plata, and up the Uruguay, and from 

 the upper Parana drainage in Sao Paulo. The species has not been found in the 

 lower and middle Parana, although it should be expected there. In addition it is 

 found, as our specimens show, in the drainage of a coastal stream in Rio Grande do 

 Sul (Guahj'ba). This is a remarkable fact, but it should be borne in mind that the 

 head-waters of the Ibicuhy (Uruguay drainage) and those of the Jacuhy (Guahyba- 

 drainage) closely interlock, and that stream piracy in this region is quite probable. 



Characters of the Shell. — Shell rather small (maximum length 44 mm.), moder- 

 ately thick, outline more or less subcircular or subpolygonal. Height 77 to 88 pr. 

 ct. of length. A'alves very little gaping in front, sometimes almost closed. Dorsal 

 margin straight behind the beaks, or gently convex, much lower in front of the 

 beaks, and straight here or even somewhat concave, passing in a curve or a very 

 blunt angle into the anterior margin. Posteriorly the dorsal margin passes in 

 a distinct, or indistinct, blunt angle into the posterior margin. The latter descends 

 obliquely, and is straight, rarely slightly convex or concave. It passes in a curve 

 into the posterior part of the lower margin, forming the blunt posterior end of the 

 shell. The lower margin is curved, the strongest part of tlie curve and the lowest 

 . point of the shell is situated at 54 to 60 pr. ct. of the length from anterior end, and 

 is thus immediately behind the middle. From this point the lower margin curves 

 up rather strongly in the posterior jiart. In the anterior portion it is very gently 

 curved, or almost straight, and finally curves up sharply into the anterior margin. 

 Thus the anterior end of the shell appears narrower than the posterior, and the 

 shell has the shape of an irregular pentagon. 



Valves very gently convex, rather compressed, greatest convexity behind the 

 middle of the shell, without a distinct posterior ridge. Sides not flattened. Toward 

 the anterior and posterior end the shell is somewhat compressed. Diameter 41 

 to 51 pr. ct. of the length. Beaks little swollen, and very little prominent, located 

 at 31 to 39 pr. ct. of the length. Lunula narrow and short. 



Epidermis dull, not shining, with very crowded, irregular concentric lines, 

 which form fine lamellee, and give a cloth-like appearance to the surface. But 



