ortmann: south American naiades. 605 



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curving sharply around into the lower margin, thus forming the posterior point of 

 the shell. This point is only a little elevated above the base-line, since the pos- 

 terior part of the lower margin runs straight forward, being almost parallel to 

 the dorsal margin (there is some variation in this respect; on the average these 

 two margins diverge very slightly towards the front). From a point at, or slightly 

 behind, the middle of the shell the lower margin changes its direction abruptly, 

 running forward and upward, thus forming a lower blunt angle at about the middle 

 of the shell. The ascending antei'ior part of the Iowxt margin is first almost straight, 

 but then it passes in a curve into the anterior margin, which is narrowly rounded. 

 The shell appears slightly narrower anteriorly, with the greatest height in the pos- 

 terior part. The taper of the i)osterior end is stronger than in A. trigona, and thus 

 the shell appears somewhat oblique. 



Valves more convex than in .4. trigona, diameter 38 to 46 pr. ct. of the length. 

 The greatest swelling is towards the beaks, but the beaks are not much elevated 

 above the hinge-line, so that they appear rather depressed. Location of beaks 

 at 27 to 31 pr. ct. of the length of the shell. The convexity of the valves is like 

 that of A. Irigona, compressed upon the posterior slope, and flattened in front of 

 the posterior ridge. Posterior slope with one or two more or less distinct radial 

 ribs, accompanied by shallow depressions. 



Epidermis similar in sculpture to that of .4. irigona, but more frequently 

 with fine scalariform stripes, chiefly upon the anterior part of the shell, producing 

 the appearance of fine radial sculpture. Color of epidermis from dark or light 

 greenish and yellowish to light or dark brown; one specimen inclines more toward 

 olive-green. Most of the specimens are somewhat concentrically banded with 

 lighter and darker color. 



Hinge-line straight behind the beaks, inclining downward in front of them, 

 but very little so in the youngest specimens. Ligamcntal sinus triangular, shaped 

 like that of ^4. trigona. 



Cavity of shell moderate, that of beaks somewhat deeper. Nacre whitish and 

 iridescent, with indistinct radial stria?. Prismatic zone narrow, subconcentric 

 to the margin. Muscle- and mantle-scars as in .4. trigona. 



