ortmann: south American naiades. 



575 



these lamelliE may be worn off in part, and the surface may be smootlicr. No 

 distinct radial sculpture is visible, but there may be a trace of a radial rib ujwn the 

 posterior sloi)(\ Color of epidermis blackish gre(>n, without color-markings, ex- 

 cept traces of one or two dark rays ujjon the posterior slope. 



Hinge-line straight behind the beaks. In front it is curved down more or 

 less suddenly, and carries a single pseudocardinal tooth in each valve. The tooth 

 of the left valve stands in front of, and somewhat below, that of the right valve. 

 Both teeth are vertically depressed, somewhat siioon-shajied, and jjroject under 

 the edge of the opposing valves into corresponding shallow grooves. Ligamental 

 sinus triangular, not deeper than wide; its anterior margin more or less oblique to 

 the hinge-line. 



Nacre very iridescent, whitish, but generally with salmon, pink, or pale purple 

 stains, chiefly in the cavity of the shell. Irregular radiating lines present, but 

 only near the margin. Nacreless (prismatic) zone of irregular width, narrow be- 

 hind, suddenly increasing in width at the lower point of the lower margin, and re- 

 maining wide to near the anterior margin. Greatest width of this zone over half 

 the distance of mantle-line from margin. Color of nacreless zone dull olive, gray- 

 ish, or grayish red. 



Cavity of shell and beaks shallow. Anterior adductor-scar well marked, ovate. 

 Anterior retractor-scar small, but distinct, separated from or connected with the 

 adductor-scar, hing in the left valve on the base of the pseudocardinal tooth. An- 

 terior protractor-scar connected with adductor-scar. Posterior adductor-scar 

 faint, ovate; posterior retractor-scar forming a triangular upper projection of 

 adductor-scar. No dorsal scars. Pallial line subconcentric to the margin. 



Measueements. 



Remarks. — These measurements agree well with those given by Von Ihering 

 and Nehring. According to Von Ihering the height ranges from 80 to 84 pr. ct. of 

 length, the diameter from 46 to 51 pr. ct., while Nehring gives for the height 77 to 82 

 pr. ct., the diameter 46 to 49 pr. ct. Lea's original figure shows, according to 

 Von Ihering, 80 pr. ct. for the height, and 44 \)r. ct. for the diameter. The sped- 



