ortmann: south American naiaues. 599 



rather thin when youiiji;, ^sliglitly more solid when older, hut never of considerable 

 thickness. Outline subcircular, huf a little irregular and variable, and distinctly 

 oblique. Height 80 to 93 pr. ot. of tlu; length. Valves not gaping. Dorsal 

 margin nearly straight. Posterior and anterior margins uniting with the dorsal 

 in blunt angles. Posterior margin obliquely descending, first straight, then curved, 

 and passing in a rather regular curve in the posterior part of the lower margin, 

 without any trace of a posterior point. Lower margin ascending forward more 

 strongly, and curving up into the anterior margin, so that the anterior part of the 

 shell appears somewhat narrower than the posterior part, thus producing the 

 obliquity. 



Valves inflated, diameter 49 to 56 pr. ct. of the length. Beaks somewhat 

 swollen and inflated, elevated above the hinge-line and incurved, their tips im- 

 mediately above the hinge-line in the young, a little higher in older shells. Loca- 

 tion of beaks at 33 to 44 pr. ct. of the length. Outer surface of shell rather regularly 

 convex, but anterior and posterior slopes somewhat com^Dressed. Posterior ridge 

 quite indistinct. 



This species is remarkable for the presence of beak-aculpturc, which is generally 

 absent in this subfamily. However, I think that this sculpture is not genetically 

 connected with that of other Naiades, but probably is independently developed. 

 Quite a number of my younger specimens show it. The very tip of the beak ap- 

 pears as a small tubercle, and is succeeded by three to five concentric bars, which 

 follow the growth-lines, and are low and rounded, but perfectly distinct in the 

 middle, disappearing anteriorly and posteriorly. These bars are restricted to and 

 crowded together at the extremity of the beaks, and disappear at a short distance 

 (3 to 4 mm.) from them. 



Epidermis with fine and crowded, concentric, somewhat anastomosing lines, 

 which become lamellarly elevated toward the margins, and in well preserved shells, 

 chiefly young ones, they have this character all over the shell. In addition there 

 may be fine and faint radial striae, but there are no scalariform stripes. In old 

 and partly worn shells the surface becomes rather smooth, but remains alwaj's dull, 

 and is not shining. Color of ci)idermis from dark green to yellowish brown. The 

 normal color in young specimens seems to be lighter or darker green, sometimes 

 with indistinct dark green rays (seen only in transmitted light) . In older specimens 

 the color becomes greenish brown to light browni, due to partial abrasion of the 

 epidermis. There are always two more or less distinct dark green or blackish 

 rays upon the posterior slope, often accompanied by two lighter, yellowish rays. 

 Larger specimens maj' have a few dark brown growth-rests. 



