ortmann: south American naiades. 481 



from D'Orbigny with the al)ovr locality; but it is finite possible that a mistake 

 was made, since D'Orbigny also collected in the Amazon system in Bolivia, in 

 the general region, whence our specimens come. Nobody else ever re-discovered 

 this species in the La Plata, although frequent collections have been made. Our 

 material is beyond any doubt this species, and the locality is authentic. 



The set, although containing only six specimens, comprises young and old, 

 males and females, and thus it is worth while to giv(> a full description. 



Description of the Shell.— Of medium size, growing larger than any of the 

 preceding species (maximum length 58 mm.; the type is 42 mm., according to 

 Simpson), rather solid. Outline subelongated, subelliptical, or long-ovate, rounded 

 in front, pointed behind. Height 41 to 54 pr. ct. of length. Valves not gaping. 

 Dorsal margin straight in young specimens, very gently curved in older ones. 

 Anteriorly the dorsal margin forms an indistinct angle with the anterior margin 

 in young individuals; in old ones it passes into it gradually. Posteriorly the dorsal 

 margin forms a very obtuse angle with the posterior margin, or, in the largest 

 specimen, i)asses gradually into it in a gentle curve. Posterior margin obliquely 

 descending, gently curved, meeting the posterior portion of the lower margin in a 

 blunt, but distinct, point, which is elevated above the base-line, but nearer to the 

 latter than to the line of the ujipcr margin. Ventral margin gently and rather 

 regularly convex, its lowest point slightly behind the middle of the shell, ascending 

 in front and behind it. In front it curves up into the anterior margin. Thus the 

 shell has a long-ovate, almost subeUiptical outline, with the anterior end only 

 slightly narrowed and rounded, and the posterior more tapering and bluntly pointed. 



Valves moderately convex, convexity rather uniform all over the disk, but 

 strongest near the beaks and upon the umbonal ridge, which forms a rounded, but 

 rather distinct, angulation running toward the posterior point. Posterior slope 

 somewhat compressed. Diameter 23 to 36 pr. ct. of length. Beaks not swollen, 

 hardly elevated above the hinge-line, located at 18 to 25 pr. ct. of the length. Since 

 the shell is more swollen towards the beaks, the latter appear depressed, chiefly 

 in old specimens. Beak-sculpture distinct and well developed, consisting of about 

 sixteen radial bars; the anterior bars are shorter (about 5 mm. long), the posterior 

 much longer, chiefly upon the umbonal ridge, where they are 15 mm. long, or even 

 more. There are two systems of these bars; eight or nine anterior, and seven or 

 eight posterior bars. In the middle of the shell one or two pairs unite at a sharp 

 angle, and sometimes a short, odd bar stands between the innermost pair. The 

 anterior bars are sharp; the posterior ones are also rather sharp near the beaks, 

 but towards their lower ends they gradually become thicker and i-ounded, broaden- 



