ortmann: south American naiades. 613 



Rio Paranu up to sixty miles above Corricntes, Arji;entiiia (D'Orl)igny, niem- 

 branaeea) . 



Localities Represented in the Carnegie Museum. — Rio Parana (Hartman collec- 

 tion). One specimen. In pond along banks of Rio Negro, Santa Isabel, Uruguay 

 (J. D. Haseman coll., February 11, 1909). One specimen. Arroyo Migueletc, 

 Montevideo, Uruguay (J. D. Haseman coll., February 17, 1909). One specimen. 

 Rio de la Plata, San Isidro, 20 km. N. of Buenos Aires, Argentina (A. Windhausen 

 coll., January, 1917). Twenty specimens with soft jxarts, males and females. 



Distribution.— La Plata system, from the mouth near Buenos Aires up the 

 Parana to the province of Corrientes in Argentina; also in the Rio Uruguay and the 

 Rio Negro, and the tributaries of the La Plata in the Banda Oriental of Uruguay. 



Characters of the ,S/«'ZL— Shell rather large (length up to 100 mm. and over), 

 rather thick and solid. Outline very oblique, subovate, longer or shorter, height 

 60 to 74 in-, ct. of the length. Young shells probably higher and more subrotund, 

 according to the growth lines in older shells, but such shells have not been observed 

 (the smallest shell is 75 mm. long). Valves not gaping. Dorsal margin straight 

 or gently curved, generally with a well marked, blunt posterior angle, while the 

 anterior angle is less distinct, often quite rounded. Posterior margin obliquely 

 descending, generally almost straight, rarely gently convex, and very rarely a 

 little concave, curving broadly and regularly into the posterior lower margin, 

 generally forming no angle or posterior point, but sometimes with a trace of it- 

 The anterior part of the ventral margin is strongly ascending and always more or 

 less gently curved, and often almost straight, curving up into the anterior margin, 

 which is narrowly rounded. Thus the anterior end of the shell appears much nar- 

 rower than the broadly rounded posterior part, rendering the shell decidedly oblique. 



Valves moderately swollen, diameter 38 to 44 pr. ct. of length, rarely slightly 

 less. Beaks slightly convex, and only little .elevated above the hinge-line, located 

 at 28 to 34 pr. ct. of the length. Disk moderately convex, more strongly so over 

 the middle part and the posterior ridge; posteriorly compressed. Posterior ridge 

 very indistinct, but often there is a radial rib upon the posterior slope, sometimes 

 accompanied by a furrow. The anterior end of the shell is also slightly compressed. 

 Epidermis rather smooth, but with fine, irregular, concentric striae, almost 

 effaced in the middle of the disk and toward the beaks, more distinct toward the 

 margins, where they may become sublamellar. Yery fine radial, ol)lique, or 

 reticulated wrinkles may be present between the striff. Scalariform radial stripes 

 are generally absent, but in some specimens slight traces of them are seen. Color 

 of epidermis dark greenish-olive to various shades of brown. Generally there is 



