ortmann: south American naiades. 503 



ments of the dimensions of tho two sots largely overlap, and they agree very well 

 with those given l)y \'on Ihering. Of course, my material being more plentiful, 

 the range of the dimensions is wider. 



An apparently allied form is D. suppoHitus Simpson (1914, p. 1245; IMarshall, 

 1917, p. 385, PI. 51, fig. 2; PI. 54, fig. 1-4)."'' The type is, according to Marshall, 

 from "Parana, Brazil," and other specimens are from Rio Tietc, Sao Paulo, and 

 other localities in southern Brazil. This is also a comparatively small form, but, 

 according to the measurements given, it is more elongated, the height being in 

 the type 52 pr. ct. of the length (53 pr. ct. according to Marshall), and in another 

 specimen 58 pr. ct. This differs somewhat from our s])ecimens (58 to 67 pr. ct.) 

 and from Von Ihering's paulida (59 to (j5 pr. ct). In addition suppositus has a 

 chestnut-bronzy epidermis, while it is greenish in paulista. Thus the two forms do 

 not agree. 



Analomij. — I have investigated the soft parts of all of my shells, and there 

 were altogether, aside from several small ones, where the sex could not be ascer- 

 tained, nineteen males, six barren females, five gravid females with eggs, and seven 

 gravid females with glochidia. 



The anatomy in general is like that of the genus Diplodon. But in this case 

 again the marsupium (PI. XLVI, fig. 1) in its size and location shows specific peculi- 

 arities, exhibited by all of my females. It agrees with the preceding species in 

 the fact that it occupies only a part of the inner gill, and in the slightly anterior 

 location. But the marsupial part is very small, occupying about one-fifth or one- 

 fourth of the length of the gill, leaving a considerable portion non-marsupial at 

 the anterior end, and half or nearly half at the posterior end. The figure on PI. 

 XLVI, fig. 1, represents a specimen with a rather small marsupium; generally it 

 is a little larger. Thus the marsupium is in the middle of the gill, and slightly 

 in advance of the middle. When charged, it forms here a rounded or oval swelling, 

 rather distant from the base, and extending not quite to the edge. There is a 

 slight variation in the specimens from the two main localities. In those from Mogy 

 das Cruzes it is distinctly in front of the middle; in those from Mogy Mirim more 

 median, but this difference is very slight and not always distinct. The inter- 

 laminar connections form interrupted septa (also seen in the section, PI. XLVIII, 

 fig. 1), and there is no distinctly reticulate arrangement. 



Glochidhim (Text-fig. 4/, p. 4()9) rather large, subtriangular, oblique, with 

 hooks. L. 0.32, H. 0.27 mm.; hooks about 0.10 mm. long. (Thus the glochidium 

 is slightly larger than in any of the preceding species.) 



"The name suppositus was first given by Von Ihering (1893, p. 102) without description, from 

 Rio Grande (Upper Parand, boundary between Sao Paulo and Minas Geraes). 



