ortmann: south American naiades. 543 



Other Localities. — Rio Uruguay (D'Orhigny) (Lea, nocturnus) ; Rio Parana up 

 to above Corrientes (D'Orbigny); Rio Paraguay (Von Ihering, 1893, p. 119); 

 Rio Paraguay, 25° S. Lat. (Von Martens) (This is near Asuncion, Paraguay). 



New Localities. — Rio Paraguay, Corumba, Matto Grosso, Brazil (H. H. 

 Smith coll.). One large complete shell, one small left valve. In swamp of Lam- 

 bare, five miles below Asuncion, Paraguay (J. D. Haseman coll., March 31, 1909). 

 One right, one left valve. Rio de la Plata, San Isidro, Argentina (20 km. North of 

 Buenos Aires) (A. Windhausen coll., January 1917). Five specimens with soft 

 parts, males and barren females. 



Distribution. — Rio de la Plata, Parana, and Paraguay, from near Buenos 

 Aires up to Matto Grosso, Brazil; Rio Uruguay. 



Description of Shell. — Shell solid and rather thick, large (length up to and 

 over 100 mm.). Outline angularly rounded, or broadly ovate, little oblique. 

 Height generally over 80 pr. ct. of the length, rarely less (lowest figure known 76 

 pr. ct., but this stands rather isolated, being found in a specimen measured by Simp- 

 son). Valves closed, or very little gaping in front. Dorsal margin gently curved 

 or straight (when j'oung), descending anteriorly and passing into the anterior 

 margin, without -forming a distinct angle. Posteriorly the dorsal margin forms 

 a more or less distinctly rounded angle with the posterior margin. The latter 

 descends obliquely and is straight, or even a little concave; then it curves around 

 in a sharp curve into the postero-ventral margin, this curve forming a blunt posterior 

 point of the shell, well elevated above the basal line. The ventral margin has a 

 sharp curve near its middle, forming a blunt, projecting angle at, or a little behind, 

 the middle of the shell. The highest part of the shell is at 50 to 60 pr. ct. of the 

 length. This ventral angle is generally distinct, but may be indistinct. From 

 this projection the lower margin slopes upward in either direction, backward and 

 forward, but the anterior part ascends more strongly, and is longer than the pos- 

 terior, and is often almost straight jiart of the way, before it finally curves up into 

 the anterior margin. The shell thus appears somewhat narrower anteriorly than 

 posteriorly, and has a rounded pentagonal shape, the five angles being formed by: 

 1, the l^eaks; 2, the upper posterior angle; 3, the posterior end; 4, the middle of the 

 lower margin; 5, the anterior end. 



Valves not much swollen, moderately convex over the disk, but slightly flat- 

 tened on both sides of a submedian more convex ridge. The latter runs toward 

 the projection of the lower margin, but is very faintly marked. Posterior ridge 

 rather distinct on account of a distinct radial depression behind it. This depres- 

 sion makes the posterior slope much compressed, like a narrow wing. The slight 



