ortmann: south American naiades. 



581 



Measurements. 



Remarks. — The measurements for minuana given by D'Orbigny in the text and 

 those taken from his figures do not agree, but according to the latter the height 

 is about 64 to 67 pr. ct. of length, and the diameter about 50 pr. ct. Our specimens 

 from Caehoeira have, on the average, a smaller diameter (falling as low as 45 pr. 

 ct.), and in this respect they are transitional toward the variety parchappi; but in 

 other characters (beaks and hinge-line) they agree better with typical minuana. 

 M. pazi Lea is apparently the same species. Simpson unites it with parchappi, 

 and it indeed approaches the latter in obesity, having a diameter of 46 pr. ct., 

 {parchappi of D'Orbigny has 43 pr. ct.), but here again the beaks and the curvature 

 of the hinge-line are more like minuana, and for this reason I place it here. 



Just because such intergrades do exist, I regard parchappi as a variety of 

 minuana (See below). Where the line between the two forms should be drawn re- 

 mains doubtful, and naturally it could not be expected that there is a sharp line. I 

 have named as miuana my specimens with the diameter of 45 pr. ct. and over, 

 because their beaks and hinge-line are more like those of minuana, but this procedure 

 possibly may require modification, when more material is studied. 



Anatomy.— I have examined the soft parts of six males, and of five females. 

 Two of the latter, collected February 5 (Uruguayana), were gravid. 



Lea (Obs. XII, 1869, p. 273) has described the soft parts of M. pazi, and, as 

 far as it goes, this description agrees with our specimens. 



The structure of the soft parts is essentially like that of the foregoing species 

 {lentifonnis and paraguayana). It should be remarked that the paljn are rather 

 large, with a somewhat longer posterior truncation, and the posterior margins are 

 connected at the base. Anal opening practically smooth. Papilla of the branchial 

 opening very small. 



In the gravid female the whole inner gill is charged, with exception of the 

 outermost extremities. The swelling of the mar.vipium is very moderate, and in a 

 cross-section (PL XLVHI, fig. 76) it is seen that only the part of the septa, extend- 



