ortmann: south American naiades. 



o33 



elovatod, showing this character all over the disk, being, however, more distinctly 

 lamellar and more crowded upon the posterior slope and near the margins. Even 

 when these fine lamella? are worn off, the epidermis does not become shining, but 

 remains dull, and when well-preserved, the epidermis a[)pears cloth-like. There 

 are no traces of radial sculpture. Color of epidermis dark greenish black, but 

 often in the middle of the disk and towards the beaks brownish black. The greenish 

 tint is not very evident, and is best seen in young specimens. 



Hinge-line very gently curved. Ligamental sinus over the posterior tiiird of 

 the laterals. Lateral teeth curved, one in right, two in left valve, their edges 

 slightly corrugated or nearly smooth. Pseudocardinals narrow and compressed, 

 but not very long, corrugated and rugose, but not cut up, two well develoi)ed 

 pseudocardinals of nearly equal size in right valve, one well-developed in left valve; 

 but there often is a smaller posterior one in the left valve, and another small, low, 

 and narrow anterior one, so that the left valve may have three pseudocardinals. 

 The middle one, however, is always the largest. Sometimes the posterior pseudo- 

 cardinal of the right valve is higher and thicker than the anterior. 



Cavity of shell and beaks shallow. Nacre blueish silvery, very iridescent, 

 and sometimes discolored yellowish. Anterior adductor-scar distinct and moder- 

 ately impressed. Anterior retractor-scar rounded, small and rather deep, separ- 

 ated from atlductor-scar. Anterior protractor-scar united with it. Posterior 

 adductor-scar rather indistinct and not impressed, the posterior retractor-scar 

 forming an upper process of it. Pallial line not very sharp. Dorsal scars few in 

 beak-cavity. 



Measurements. 



Remarks. — The dimensions of this species change slightly with age, in that 

 younger shells are not so high in proportion, and consequently, the.y appear 

 less distincjtly oblique than older ones. The diameter of young shells is also less 

 than in old ones, and the highest figures (37 to 45 pr. ct.) are shown by old females. 

 In other respects my specimens are rather uniform, and the oblique, not very 

 elongated shape, and the dull, blackish eiiidermis and silvery nacre are chiefly 

 characteristic. 



