ortmann: south American naiades. 603 



very gently eurved, and thi'n it curves up into the anli-rior margin, which is narrowly 

 rounded. The anterior part of the shell does not appear narrower than the pos- 

 terior. The greatest height of the shell is a little behind the middle, and anteriorly 

 it rather gradually becomes narrower, while posteriorly it tapers very decidedly 

 to the posterior point. Thus there is also no marked oblicjuity in the shell. (In 

 the young specimen from Santa Rita the posterior taper is not so strong). 



Valves moderately convex, diameter 34 to 30 pr. ct. of length. Beaks not 

 very prominent above hinge-line, located at 18 to 20 pr. ct. of the length. Con- 

 vexity of the valves greatest over the posterior ridge, least between this ridge and 

 the region of the lower angle of the lower margin. Posterior slope compressed, 

 but hardly any comi3ression at anterior end. A distinct radial rib upon the pos- 

 terior slope, b'ing between two radial depressions; sometimes there is a trace of a 

 weak radial rib above it, Init generally this is not very distinct. In young specimens 

 the radial rib is indistinct. 



Epidermis rather smooth, with irregular concentric lines, which become 

 lamellar uj^on the posterior slope and towards the ventral margin. No distinct 

 radial sculpture, except some irregular and rather fine scalariform stripes. Color 

 of epidermis normally a very dark green upon the disk, which, however, may turn 

 to brown. The posterior slope may also be dark green, with the shallow grooves 

 brownish, or it may be entirely brown, and in the young specimen it is brown, 

 with the radial rib marked by a rather distinct dark green ray. The disk has no 

 color rays. 



Hinge-line practically straight behind the beaks, and in front of them it 

 gently curves down. Ligamental sinus triangular, about as deep as wide, its an- 

 terior margin vertical to the hinge-line, or slightly descending backward. 



Cavitj' of shells and beaks moderate. Nacre whitish, more or less iridescent; 

 only in the young specimen with irregular and indistinct radial stride. Prismatic 

 border rather narrow, subconcentric with margin, and nowhere noticeably widened, 

 except very slightly so in the region of the projecting part of the lower margin. 

 Anterior adductor-scar well impressed, subovate, united above with scar of an- 

 terior retractor. Anterior protractor-scar united with or separated from that 

 of the adductor. Posterior adductor-scar faint, subovate, the posterior retractor- 

 scar forming a triangular upper pi-ojection of it. Pallial line subconcentric to the 

 margin. 



According to Wagner, the height would be 57 \n\ ct. of the length. 



Remarks. — This species has a quite characteristic shape, which varies only 

 slightly, and the most prominent features are the blunt angle of the lower margin, 



