ortmann: south American naiades. 521 



golden brown, without green. Also the epidermis is not at all "cloth like," as 

 described in trivialis (when fresh), and the description of the pseudocardinals of 

 the left valve does not agree at all. There are said to be two pseudocardinals, 

 the anterior one sometimes feeble, while in our specimens there is generallj' only 

 one well-developed, and this is the anterior, and if there is a smaller second pseudo- 

 cardinal, this is the posterior. The figures of trmalis given by Marshall (1917, 

 p. 386, PI. 54, figs. 5-8) show also that the outline is different, being evenly rounded 

 behind. Among my numerous specimens there is not a single one which shows this 

 character, and thus I cannot identify them with D. trivialis. 



Description of Shell. — Of moderate size (ma^cimum length 68 mm.), rather 

 solid. Outline short subelliptical or subovate, or subrhomboidal, when young. 

 Height from 62 to 74 pr. ct. of length. Upper margin nearly straight when young, 

 more or less curved when old, in the first case forming an angle with the obliquely 

 and rather steeply descending posterior margin, in the latter case passing into it 

 more or less gradually. Posterior margin gently concave, straight, or gently 

 convex, forming a rounded angle with the lower margin, which may be more dis- 

 tinct in older shells. This posterior point is more or less elevated above the base- 

 line. Lower margin gently and regularly curved, in older shells more nearly 

 straight in the middle. Anterior end of shell slightly narrower than the posterior 

 in young shells; in old shells this may be reversed. Thus the shell is, when young, 

 more subrhomboidal, with an upper posterior angle (somewhat subalate), and, 

 when old, the shell becomes subelliptical or subovate, with the posterior end a 

 little tapering. 



Valves rather regularly and evenly convex, sides not distinctly flattened. 

 Greatest diameter a little anterior to the middle. Posterior ridge present, but 

 rounded and indistinct, often (chiefly in young specimens) marked by a shallow 

 radial groove running down the posterior slope, which thus appears as compressed 

 and slightly elevated toward the upper-posterior margin (subalate). Diameter 

 32 to 42 pr. ct. of the length, so that the shell is rather compressed. Beaks not 

 inflated and not much elevated, located at from 25 to 31 pr. ct. of the length. Beak- 

 sculpture consisting of fine and short radial bars, hardly more than 5 mm. long, 

 fifteen to eighteen in number, those in the middle converging at their lower ends 

 (one or two pairs). They are hardly longer upon the posterior ridge, and not ap- 

 preciably thicker. A few obli(iue wrinkles may be present upon the posterior 

 slope. In most cases the beak-sculpture is entirely obliterated by erosion of the 

 beaks, and in general it is fine, short, and poorly develojied. Lunula present, 

 narrow in young specimens, wider in old ones, but very variable. 



