ortmann: south American naiades. 607 



ever, just the opposite is the case, mortnninna beiiij^ more swollen (40 \)v. ct.) than 

 weddelli (33 pr. ct.). The second difference is not at all correct according to the 

 published figures; and the third apparently is founded only ujion the slight indica- 

 tion of the muscle-scars in Lea's figiu-e, and is not ess(>ntial, anyhow. Besides, 

 Hup6 says that the specific difference is supported by the different distribution 

 of the two forms. This again is not evident. A. weddelli is fi-om the region of 

 the divide between the Amazon and Paraguay-drainages, and it maj' very well be 

 from the latter. One of our localities is not very far from it. A. morloniana and 

 weddelli, indeed, have been united already by Von Ihering (1893, \). 118). 



A. lingxdata differs from the others only in size and the color of the epidermis. 

 The latter, on the plate, is dark green, while the text says that itis blackish brown, 

 and thus we cannot rely on it. The shell of lingulata is more regularly elliptical, 

 but this is not astonishing when we consider the greater age of this shell. It is 

 also from the same general region (upper Paraguay) as morloniana and weddelli. 



Very similar forms are found also in the Amazon-drainage: castelnaudi Hupe, 

 solidula Hupe, amazonensis Lea, and elongala Swainson. But these cannot be 

 united with niortoniann, since they all are more elongated. 



Characters of the Shell. — Shell quite thick and solid. Outline subovate or 

 nearly subelliptical, bluntly pointed behind, lower margin gently convex. Height 

 61 to 63 pr. ct. of the length, falling, in old specimens, as low as 53 pr. ct. Valves 

 not gaping. Dorsal margin gently convex; the part behind the l:)eaks may be 

 almost straight; in front of the beaks it descends more or less. Posteriorly the 

 dorsal margin forms a blunt angle, or may pass almost insensil:)ly into the posterior 

 margin. There is no distinct angle anteriorly. Posterior margin obliquely descend- 

 ing, more or less convex, curving into the lower margin and forming with it a dis- 

 tinct, but rounded, posterior point of the shell, which is situated at a certain eleva- 

 tion above the base-line, but nearer to the latter than to the line of the upper margin. 

 Lower margin gently and rather regularly curved, ascending somewhat toward 

 the posterior end of the shell, and more strongly so in its anterior part, where it 

 passes in a regular curve into the anterior margin. The anterior part of the shell 

 is only slightly narrower than the posterior, which is wid(>st (highest) a little behind 

 the middle of the shell, and then tapers gently toward the posterior point. The 

 shell is thus transverse, and hardly oblique. 



Valves moderately convex, diameter 37 to 41 pr. ct. of the length {iveddelli is 

 more compressed, 33 pr. ct.). Beaks moderately convex, not very prominent above 

 hinge-line, at 23 to 25 pr. ct. of the length, that is to say, rather anterior. Convexity 

 of valves rathei- uniform all over the disk, strongest over the posterior ridge, slightly 



