ortmann: south American naiades. 



477 



of which the seventh and eighth, or ninth and tenth, unite in the middle of the 

 valve in a sharji angle ; sometimes a short odd bar is found between these pairs. 

 The bars are rather fine and sharp in front, but those behind, near the umbonal 

 ridge become broad and rounded, their lower ends being irregular and indistinctly 

 tubercular (much less so than in D. hylwus) occasioned by concentric lines cutting 

 across them. These posterior bars also are distinctly longer than the anterior 

 ones. Sometimes there are a few additional fine bars near the beaks on the posterior 

 slope and below them a few oblique wrinkles. 



The posterior end of the shell is also somewhat variable, but it never is rounded 

 and slightly biangular, as it is in D. hylccus. The posterior margin is obliquely 

 descending, and the lower posterior end is bluntly pointed, the angle being more 

 or less prominent. The lower margin of the shell is evenly convex in some speci- 

 mens, in others it is more strongly convex a little back of the middle, forming a 

 blunt angle. 



The specimen from Corumbd is young, and the beak-sculpture covers all of 

 the shell. It is also a httle more compressed than the others, but agrees with them 

 in all other characters. 



Measurements. 



Thus my specimens are on the average slightl}' higher and shorter, and some- 

 what more swollen than the original, unless there are inaccuracies in the figure, 

 which is not impossible, since the figure is 32 mm. long, while the text gives the 

 length as 21 mm. 



Anatomy. — The only specimen (S. Luis de Caceres) of which soft parts are 

 at hand, is a male, and has the structure typical of the genus. The anal open- 

 ing is slit-like, and about three-fourths of the length of the branchial. The 

 branchial opening has fine papillse. The palpi arc triangular and rather small, 

 the posterior margins are not connected. Inner lamina of inner gills connected 

 with abdominal sac. Gill-structure typical. 



