ortmann: south American naiades. 537 



Remarks. — According to Simpson (1914), liiis form (yralus) is a variety of 

 D. fontainianus (D'Orl)igny), and there is no doubt that both are closely allied to 

 each other. However, Simpson erroneously gives the distribution of fontainianus 

 as: "Uruguay River and its affluents; Parana River, southern Brazil," while it 

 is known onl^^ according to D'Orbigny and Von Ihoring (1893) from the Rio 

 Parahyba, Rio de Janeiro, the upper Parana-drainage in Sao Paulo, and Lagoa 

 Santa in Minas Geraes (Rio de las Velhas, Sao Francisco-drainage). In 1910 (p. 

 138) Von Ihcring drops this latter locality. Fontainianus has never been reported 

 from the Uruguay. Thus it is not very likely that my specimens from this river 

 are D. fontainianus, although they agree with it in some particulars. 



According to Von Ihering's account of D . fontainianus (1893, p. 90, PI. 4, fig. 6), 

 the latter differs from my specimens chiefly in having the posterior retractor- 

 scar separated from the adductor-scar, but otherwise they are very similar. This 

 holds good chiefly in the color of the epidermis. My specimens are greenish black, 

 which agrees best with Von Ihering's D. fontainianus, which is "black, with greenish 

 basis," and to a degree with D'Orbigny 's "brun noiratre," while Lea describes his 

 gratus as "dark olive brown." The differences between D. gratus and D. fontaini- 

 anus, as given liy Simpson, concern the shape of the shell, which is said in gratus 

 to be more wedge-shaped in front, and the texture of the epidermis, which is said 

 to be smoother, subshining, and lighter in color. These differences I cannot 

 recognize in my specimens. It is therefore hard to decide by which name they 

 should be called. However, my specimens coming from the Uruguay River, 

 being practically topotjq^es of gratus, I have concluded to call by this name, and, 

 in order to avoid any misunderstanding, I have given a full description of them. 



Von Ihering (1893) believes that he has discovered sexual differences in the 

 shell of D. fontainianus, the males being higher and shorter, and more rounded, 

 the females being longer and more subtrapezoidal. Similar differences in shape 

 are also noticed in our D. gratus. I have seventeen specimens in which the sex has 

 been positively ascertained by examination of the gills, and I find that it is not 

 possible to accurately determine the sex according to the shape of the shell. First 

 of all, the two shapes are not sharply separated, but pass insensibly into each other, 

 many specimens being intermediate between the two extremes. Besides, if form 

 has anj^ relation to sex, we may say that the highest shells (with the height of 79 

 pr. ct. of length and over) are, indeed, males; but among the others, having the 

 height of 72 to 78 pr. ct. of length, there are as many females as males, and this ap- 

 plies both to larger and smaller shells. It is true, that the average height of aU my 

 males is about 78 pr. ct., and the average height of all my females is 75 pr. ct.; but 



