ortmann: south American naiades. -473 



c. Beak-sculpture woll-dcvd<i|HMl, line; ri.lKos out "P i'"" ''""' n,.<lules. . .Group of D. grunosus. 

 cc. Beak-sculpture restricted to the re«inu <,f the beaks; rulges fine, simple, not granular. 



d. Shell rather compressed and flattened upon tlu; sides, subtrapezoidal in outline, and 



u<,t distinctly pointed behind Group of D. chUcnsis. 



,ld. Shell sliKhtly or not at all compressed, convex upon the sides, mostly subelliptical or 

 subovate in outline, more fir less pointed behind. 

 e. Shell rather el.msate, not hish, subelliptical or subovate. .Croup of D. charruanu.'^. 



ee. Shell shorter and higher, subovate or subtrap(-zoidaI Group of D. lacleolus. 



aa. Shell distinctly oblique, with the longest axis forming an angle with the line of the ligament. Outline 

 subovate or subtrapezoidal, sometimes rather high and short Group of D. clUpUcus. 



1. Group of Diplodon hijlceus. 



Shell subelliptical, .subovate, or subtrapezoidal, more or less pointed posteriorly, 

 straight, not distinctly higher behind, nor oblique. Beak-sculpture weU-developed, 

 covering a considerable (but variable) part of the shell; bars rather heavy; those 

 upon, and immediately in front of, the indistinct posterior ridge of the sheU are 

 heavier and longer than the rest ; one or two of the median bars joining at their lower 

 ends; sometimes the bars are somewhat nodulous (but not granular). 



The beak-sculpture is the most essential feature of this group. Additional, 

 but subordinate, characters are found in the hinge-teeth, chiefly the larger ones in 

 each valve, which are triangular, rather thick, ragged, but not compressed. Often 

 there are two pseudocardinals in the left valve. 



I have seen the female soft parts of two species, D. hasemani and trifidus. 

 In both the marsupial part of the inner gill is located in the middle section and 

 has interrupted septa. The former species has the margined type of glochidium. 

 It seems that this group is not very primitive. 



1. Diplodon hyl^us (D'Orbigny). 

 Unio hylcea D'Orbigny, 1835, p. 36; 1843, p. 607, PI. 69, figs. 8, 9. 

 Unio hylcBus Sowerby, XVI, 18G8, PL 93, figs. 506 a and h (poor figures). 

 Margaron {Unio) hylmis Lea {pro parte), Syn. 1870, p. 31. 

 Unio hykvus Von Martens, 1894, p. 164. 

 Diplodon hylwus Simpson {pro parte), 1900, p. 884; 1914, p. 1274. 



Type-locality.— "Rio Palometas, Rio Pari, and Rio Tucabaca, in the provinces 

 of Santa Cruz de la Sierra and Chiquitos, in Bolivia." 



I have been unable to locate the first two rivers; a Rio Tucaraca, in the Chi- 

 quitos country (Prov. Santa Cruz de la Sierra in Bolivia), runs to the Paraguay 

 River, and I have no doubt that this river was intended, and it is advisable to take 

 this as the type-locality. 



