ortmann: south American naiades. 631 



margin, fonniug a blunt, but distinct point, wliicli is 1kii'(11\-, or very little, elevated 

 above the base-line. Lower margin almost straight, l)ut with a more or less distinct 

 concavity in the middle. Anteriorly the lower margin curves up into the rounded 

 anterior margin. 



Valves very slightl\- convex, practically flat ui)on the sides, and, in large 

 specimens, even with a shallow depression corresponding to the concavity of the 

 lower margin. The posterior ridge is distinct, but rounded, running towards the 

 posterior point of the shell. Above this ridge, the shell is somewhat compressed. 

 Diameter 14 to 20 pr. ct. of the length. Beaks low, and hardly elevated above 

 the hinge-line, located at 18 to 26 pr. ct. of the length, and proportionally more 

 anterior in older specimens; the large specimen, described by D'Orbigny, has 

 them at 15 pr. ct. of the length. 



Epidermis not smooth, with irregular concentric lines, heaviest upon the pos- 

 terior ridge, and with very fine striae, sublamelliform upon the posterior slope and 

 towards the margins. Faint radiating lines are present, dividing the fine striae 

 into scalariform stripes of crowded, fine wrinkles, visible only in well-preserved 

 specimens, and restricted to the anterior part of the shell. C olor of epidermis 

 greenish or brownish olive, inclining to blackish in old shells, without color-mark- 

 ings and without distinct growth-rests. 



Hinge-line straight behind the beaks, slightly descending in front of them. 

 Ligamcntal sinus much wider than deep, its anterior margin oblique to the hinge- 

 line, but more nearly vertical in the largest specimen at hand. 



Cavity of shell and beaks very shallow. Nacre, in all of my specimens, whitish, 

 but with blueish and purplish iridescence (according to other authors, it is some- 

 times coppery), with irregular radiating lines, most distinct towards the margins. 

 Prismatic zone rather narrow, and subconcentric with the margin. Anterior ad- 

 ductor-scar well impressed, subovate. Anterior retractor-scar distinct, but con- 

 nected with that of the adductor-scar. Anterior protractor-scar separated. Pos- 

 terior adductor-scar faint, subovate. Posterior retractor-scar separated from the 

 latter and rather removed from it, at least by as much as its own width; in old shells 

 up to three times its width. 



Remarks. — There can be no mistaking this species, the elongated shape being 

 so characteristic, that no other South American form could be taken for it, with 

 the possible exception of .4. falsa (Simpson) from Venezuela (See above). It 

 constitutes with the latter a peculiar group within the genus, but approaches the 

 normal Anodontites (of the cn'spato-type) more than any other. The location 



