ortmann: south American naiades. 475 



posterior ciul. Diameter of shell 33 pr. ct. of lenj2;tli. Beaks not swollen, and 

 not prominent, located at about one-fourth of the length from the anterior end. 



Beak-sculpture strongly developed, covering about half of the disk, the longest 

 bars (near the umbonal ridge) are about 20 mm. or more long. There are about 

 sixteen or seventeen radial bars, of which the ninth and tenth, and the eighth and 

 eleventh, unite in sharp angles, and between the ninth and tenth, there is a short 

 odd bar, which is indistinct on account of the erosion of the beaks. Posteriorly the 

 bars increase in length as well as thickness: the anterior bars are comparatively 

 sharji, naiTow, but much injured in my specimen; the posterior bars are broader 

 and roimded. Upon the umbonal ridge, the radial bars are again finer, and upon 

 the posterior slope there are four or five additional fine bars, which are shorter, 

 and restricted to the upper part of the slope. On the lower part of the latter, 

 there are a mnnber of fine, irregular, oblique wrinkles. The lower ends of the 

 radial bars are cut up, chiefly near tl>e umbonal slope, into irregular, low tubercles, 

 and traces of such tubercles may be seen near the lower margin of the shell. 

 No distinct lunula is seen in our shell, but this part of the shell is badly eroded. 



Epidermis with numerous, irregular, concentric wrinkles, and traces of radial 

 lines. Color brown, much like that of D'Orliigny's figure. 



Hinge-line gently curved. Ligamcntal sinus over the posterior fourth (or a 

 little more) of the lateral teeth, which are gently curved, one in the right, two 

 in the left valve. Pseudocardinals directed obliquely forward and downward, 

 two in right valve, the anterior one narrow, low, and compressed, the posterior 

 one triangular, cut longitudinally into two parts. In the groove behind this 

 tooth, the hinge-line has two small denticles. In the left valve, there are two 

 pseudocardinals, the anterior subtriangular, slightly compressed and simple, fitting 

 into the groove between the two teeth of the right valve, the posterior one broader, 

 and cut into three parts. Leaving out of sight the comparatively stumpy and 

 double character of these teeth, their finer structure can only be regarded as an 

 individual characteristic. 



Cavity of shell and beaks shallow. Nacre whitish (discolored in the cavity). 

 Anterior adductor-scar distinct, subelliptical; anterior retractor-scar separated 

 from it, small, round and deep; anterior protractor-scar connected with the ad- 

 ductor-scar. Posterior adductor-scar faint, subovatc, with an upper triangular 

 process formed by the i)osterior retractor-scar. ]\Iantle-line faint. Dorsal 

 muscle-scars in the cavity of the beaks. 



