ortmann: south American naiades. 507 



DiatrihiUion.—i^mall streams of the "Banda Oriental" in Uruguay, from 

 Maldonado westward, and also in the Rio Negro and the Rio Uruguay.'^ 



An extremely variable form, of which I possess a good set from one locality, 

 undoubtedly rejiresenting young and adult specimens of the mme species, so that 

 I am able to give a rather full account of it. It is very evident that different in- 

 dividual phases have been previously described as separate species. 



Characters of Shell. — Shell of medium size (maximum length according to 

 D'Orbigny, 70 nun.; my largest is 62 mm.), solid and rather heavy. Outline sub- 

 trapezoidal, more or less elongated, straight (not obli(iue), but very variable. The 

 upper margin may be rather straight (chiefly in young ones), or more or less curved 

 (in older ones), with or without a distinct posterior upper angle. The posterior 

 end of the shell is more or less i)ointed; the position of the point is variable, but 

 generally rather low, and little elevated above the base-line. The lower margin 

 is gently curved, often nearly straight in part (chiefly so in old shells), and never 

 curved up suddenly in its posterior part, but only gently and gradually so, if at 

 all. The posterior end of the shell is thus distinctly more tapering than the rounded 

 anterior end, the posterior point lying rather low. The proportion of height to 

 length of the shell is very variable, ranging from 48 to 65 pr. ct. 



Valves quite convex and swollen, hardly flattened upon the sides, but more 

 convex anteriorly and over the posterior ridge, which is blunt, but more distinct 

 towards the beaks (and in young shells). Diameter 34 to 50 pr. ct. of length. 

 Beaks a little inflated, but not very prominent, located at 21 to 29 pr. ct. of length. 

 Beak-sculpture seen only in my youngest specimens, extending not more than 8 

 mm. from the point of the beak, consisting of about thirteen radial bars, of which 

 only the lower ends are seen. They increase little in length posteriorly, are rather 

 sharp in front, but the longest are somewhat obtuse, while the two last, right upon 

 the posterior ridge, are again sharp and shorter. In some of my specimens, chiefly 

 the younger ones, there are some short, oblique wrinkles (one to eight) upon the 

 posterior slope; in others they are entirely absent. Lunula absent or present, 

 narrow. 



Epidermis with numerous, irregular, finer and coarser, concentric lines; the 

 finer ones sublamellar on posterior slope and towards the margin. Radial sculp- 

 ture obscure, but present in the shape of fine lines, sometimes more distinct on 

 the anterior portion of the shell. C olor of epidermis tlark olive-green to black. 



" \oiL Ilicriiig (l.S!i;{, p. 102) rcpdi-ls alliio/is fi'dii the (Uialiylm ilr.-iiiKiKC in Kio Graiulu do Siil, 

 and a variety Unracicabana) from tlie Upper Parana-draiiKigc in Sau I'aulo, but these records should be 

 doubted. 



