ortmann: south American naiades. 625 



this shape is obscured on an external view on account of the elevation of the beaks, 

 but it is best s.een from the inside, when the ligament behind the beaks is placed 

 horizontally. 



Valves moderately convex, somewhat flattened upon the sides, greatest diam- 

 eter 34 to 40 pr. ct. of the length, located upon the posterior ridge, some distance 

 behind the beaks. Posterior ridge indistinct. Posterior slope compressed, some- 

 times with traces of a radial ridge and furrow. Beaks a little inflated and a little 

 elevated above the hinge-line, their tips located at 29 to 33 pr. ct. of the length. 



Epidermis rather smooth and shining. Irregular subconcentric ridges are 

 present, but no lamelliform strise, except on the posterior slope, where they may 

 be more or less distinct, and rather crowded. Upon the smooth part of the disk 

 there are often a few radial scalariform stripes, consisting of short subconcentric 

 wrinkles. But these may be entirely missing. Coloi- of epidermis prevailingly 

 green, but shading into brown, often with concentric bands of darker or lighter 

 green and brown. Green rays may be present or absent; when present, they are 

 best develoi^ed on and just in front of the posterior ridge. Posterior slope generally 

 dark green to blackish, often with a few black rays. 



Hinge-line nearly straight behind the beaks, but gently and distinctly curving 

 down anteriorly, so that the whole hinge-line appears as gently curved. A trace 

 of a slight sinuation may be observed in a very obscure elevation of the hinge- 

 line at its anterior end, but generally this is not the case. Ligamental sinus broad 

 and deeii, in older specimens deeper than in younger ones, its anterior margin 

 running obliquely backward in young individuals, but being vertical in old ones. 

 Sometimes the anterior margin is curved, the lower point turning forwards. 



Cavity of shell and beaks moderate, corresponding to the obesity of the shell. 

 Nacre whitish and iridescent, often much discolored, and quite frequently there 

 are fine, irregular, subconcentric, waved, or zig-zag, black lines inside of the shell, 

 chiefly near the pallial line. Faint radial striations may be present in young 

 specimens. Prismatic border narrow or very narrow, subequal in width, grayish 

 green or brownish green. 



Anterior adductor-scar slightly impressed, irregular in outline, with an upper 

 process representing the anterior retractor-scar. Anterior protractor-scar separated 

 from adductor-scar. Posterior adductor-scar faint and often indistinct, the pos- 

 terior retractor forming an ujiper triangular process thereof. No dorsal scars. 

 Pallial line subconcentric to the margin. 



Remarks.— The height is given by Von Ihering as from 49 to 57 pr. ct. of length, 

 thus being less on the average than in my specimens; but I think this is due to the 



