ortmann: south American naiades. 



595 



This species may bo regarded as an elongated and narrow tenebricosfi. In 

 consequence of the general elongation, the posterior margin forms a more obtuse 

 angle with the upper margin, and the posterior end is not subtruncated, but more 

 evenly and narrowly rounded. Young specimens ai-e almost regularly long- 

 ellii)tical. The prismatic zone is comijaratively narrow. The ligamental sinus is 

 triangular, wider than deep, and its anterior margin forms an obtuse angle with 

 the hinge-line (being directed obliquely l)ackwards), or is almost vertical. (In 

 A. tenebricosa this sinus is about as deep as wide, its anterior margin is nearly 

 vertical and curved gently forwards, so that the lower point, which is quite sharp, 

 is directed obliquely forward). 



In all other characters the two species resemble each other, and this is pre- 

 eminently true of the sculpture of the epidermis. 



Measurements. 



According to Von Ihering, the height is from 43 to 51 pr. ct., the diameter from 

 22 to 33 pr. ct. of length, the beaks are at 24 to 31 pr. ct. According to Nehring, 

 the height is 41 to 43 pr. ct., the diameter 20 to 25 pr. ct. In A. tenebricosa, the 

 figures for the height are 55 to GO pr. ct. (rarely below this); for the diameter 29 

 to 45 pr. ct. and for the beaks 29 to 34 i>r. ct. 



Remarks. — The greater length of the shell is brought out by these figures, 

 and also the lesser obesity and more anterior position of the beaks, which results 

 from it. It is also seen in my specimens that the elongation is not so great in 

 younger specimens, and in the latter the measurements approach those of tenebricosa, 

 or even fall within the range of variation of it. It is possible that the two species 

 actually intergradc, a condition which has been hinted at by Nehring (p. 164) to 

 exist in the Rio Piracicaba. 



Anatomy. — The soft parts of seven males and five barren females have been 

 investigated. 



Color of soft parts whitish; inner edge of anal and branchial openings black, 

 the black color running forwards from the branchial for a certain distance. 



Anal Oldening entirely oi)en, its inner edge practically smooth, soimrated 

 from the branchial opening by a connection of the mantle-margins. Inner edge 



