ortmann: south American naiades. 529 



the shell appears iu front of it rather more compressed. Beaks not much swollen, 

 and not very prominent, located at from 23 to 29 pr. ct. of the length. Beak- 

 sculpture consists of twelve to fourteen radial bars, which are sharp and rather 

 distant from each other, the ninth, tenth, and eleventh (immediatel}^ in front of 

 and upon the posterior ridge) are longest, about 10 to 12 mm. long; the seventh 

 and eighth may consist of two bars united in V-shape, but this cannot be seen 

 clearly, since in all specimens the tips of the beaks are eroded. These median 

 bars (seventh and eighth) are also slightly shorter than those in front, and distinctly 

 shorter than those behind them; the last two or three bars are fine and somewhat 

 shorter, and stand upon the posterior slope, becoming indistinct. In a few of my 

 specimens there are traces of oblique wrinkles upon the posterior slope. Lunula 

 short and narrow, distinct only in larger specimens. 



Epidermis smooth, rather shining, with numerous, closely set, fine, concentric 

 lines, and stronger and irregular concentric wrinkles. The fine lines become sub- 

 lamellar on the posterior slope and towards the margins. A fine radial sculpture 

 is present, chieflj^ in the anterior part of the shell, but it is not very evident. Color 

 greenish black to brownish black, darkest in old shells. Young shells are more 

 distinctly greenish, dark olive-green, shading towards the beaks to gray-green and 

 brownish olive. There are no distinct color-bands and no distinct color-rays, 

 except in verj^ young specimens, where there are traces of dark green rays on the 

 posterior slope, seen when held up against a strong light. 



Hinge straight or gently curved. Ligamental sinus over the posterior third 

 or fourth of the laterals (more posteriorly in old shells). Lateral teeth curved, 

 more strongly so in their posterior part in old shells, one in right, two in left valve, 

 edges rugose. Pseudocardinals two in right, one in left valve, subcompressed, 

 not very long, in old specimens sometimes almost stumpy. The posterior pseudo- 

 cardinal of right valve stronger and more elevated than the anterior, often much 

 divided, and always much crenulated. Very often there is a second posterior 

 small pseudocardinal in the left valve. 



Cavity of shell and beaks shallow. Nacre whitish, in old specimens often 

 very thick anteriorly, and with pinkish tints; posteriorly very iridescent. Ad- 

 ductor-scars deeply impressed anteriorly, less so posteriorly. Anterior retractor- 

 scar small and deep, separated from the adductor-scar; anterior protractor-scar 

 united with it. Posterior retractor-scar connected with adductor-scar. Pallial 

 line distinct. Dorsal scars in an obliciue irregular row in the beak-cavity. 



Rcmarkfi. — This species may be what Von Ihering (1893, p. 102) calls (rtfuops 

 in the Guahyba drainage, but onl3' the remark (p. 104) that this wlhiops has a 



