ortmann: south American naiades. 



515 



Epid(MTnis shining, cliiofly so in the middle of the disk and in younger speci- 

 mens, hut with unecjutd concentric wrinkles and stritr, which become .somewhat 

 sublamellar on the posterior slope and near the margins, i^adial sculpture present, 

 consisting of irregular, often interrupt(Hl lines, chiefly upon the anterior part of 

 the shell, often appearing as "scalariform stripes" (radial rows of fine, short, con- 

 centric wrinkles). Color of ei)idermis light to dark brown; in young specimens it 

 is a beautiful, shining, golden brown, lighter in the middle of the shell and towards 

 the beaks (and sometimes here with light greenish shades). In older shells the 

 color is chestnut-brown to dark brown. In some specimens there are indications 

 of darker concentric bands. No traces of coloi--rays, except one or two very faint 

 dark rays upon the posterior slope (seen only when held up against a strong light). 



Hinge-line very gently curved. Ligamental sinus over the posterior fourth or 

 third of the lateral teeth (more posterior in old shells, which shows that the ligament 

 is comparatively longer in them). Lateral teeth gently curved, long, one in right, 

 two in left valve, finely rugose. Pseudocardinals obliquely directed forward and 

 downward, normally two in right, one in left valve, comijressed, but not very 

 long-, their edges crenulated and serrated. The posterior in th(> right valve more 

 elevated than the anterior, and often thicker. Sometimes there is a trace of a 

 second posterior tooth in the left valve, but this is always small. 



Cavity of shell and beaks moderately d(>ep. Naci-e whitish, shining, in the 

 largest specimens the thickest i)arts (toward the front of the shell) have a faint 

 rosy hue. Muscle-scars of the typical shape; the anterior adductor-scar is rather 

 deep, the retractor-scar separated from it, round and deep, the protractor-scar 

 connected with it; the posterior adductor- and retractor-scars are united. Pallial 

 line distinct. An irregular, longitudinal row of dorsal scars in the beak-cavity. 



Measurements. 



Remarks.— 1 have been unable to recognize this species in any of the published 

 descriiitions. It is a rather beautiful shell; marked by its small size, nearly 

 r(>gularly ellii)tical outline, with no obliquity, rather swollen valves, and with a 

 peculiar, shining, gold(>n bi-own color when young. It resembles, to a degree, 

 D. charnifinus, but differs from it chiefly in siz(>, more I'egular shai)(\ and the gloss 

 of the epidermis. 



