ortmann: south amkrican naiades. 



479 



specimens, and in older specimens on tlu; ])osterior slope and toward the l(jwer 

 margin. Fine, ol)scure, radial lines all over the shell. Color rather uniformly 

 dark brown, but in youn^ si^ecimens there is a distinct indication of a yellowish 

 concentric band, or the region near the beaks may be of (his color (suggesting the 

 color-pattern of I), trifidus). 



Hinge-Hne gently cui'ved. bigamciital sinus over the middle of the lateral 

 tooth, or a Httle farther back. Lateral teeth curved, one in light, two in left 

 valve. In one specimen in the right valve there is a distinct, but low, accessory 

 lateral below the normal one. As a rule two pseudo(!ardinals in each valve. They 

 are not much comjiressed and not much elongat(Ml, but rather stumpy, and are 

 very much cut up, especially the posterior ones in each valve. The anterior 

 pseudocardinal in the right valve is more distinctly compressed, but may be very 

 small or even obsolete, and thci-e may be a trace of a third pseudocardinal behind. 

 The posterior pseudocardinal in the left valve is vei-y variable in shape and size. 



Cavity of shell and of beaks moderate. Nacre in all specimens whitish. 

 Anterior adductor-scar det']), even in young specimens, irregulai'ly rovmded. An- 

 terior retractor-scar separated from it, small and deep. Anterior protractor- 

 scar united with the adductor-scar. Posterior adductor-scar less distinct, sub- 

 triangular, with a short ui>i)er process formed by the posterior retractor-scar. 

 Pallial line distinct. Dorsal scars few, located in Ix^ak-cavity and close to hinge- 

 plate. 



Measurements. 



I cannot discover any sexual differences in the shell. 



Remarks. — There is no question that this species is very closely allied to 

 D. guaranianus, and it may be described as a small, short, rather swollen D. guarani- 

 anus, with the posterior end of the shell a little sharper, the posterior ridge a little 

 more distinct, and beak-sculpture less developed. Since I possess a good number 

 of specimens of D. hasemani, and also a fair number of D. guaranianus, I do not 

 doubt the specific distinctness, since the geographical distribution also differs. 

 D. guaranianm belongs to the Parana and Paraguay Rivers, while D. hasemani is 

 from the Guaporc, tributary to the Amazons-system. However, a former con- 

 nection of these systems is indicated by the close affinity of the two species. 



