ortmann: south American naiades. 487 



The bcak-sculpturc of Simpson's species is unknown, being entirely eroded. 

 It may be that my specimens are this, but I hesitate to unite D. mimus with granosus, 

 before the beak-sculpture of the former has been described. 



3. Group of Diplodon chilensis. 



Shell rather compressed, flattened upon the sides, subelliptical, subovate, 

 subtrapezoidal, more or less elongate, not distinctly pointed behind, straight, 

 and not distinctly higher in the posterior ])art, nor oblique. Beak-sculpture 

 simple, with narrow, straight, uninterrupted radial bars, restricted to the region 

 of the beaks, and not extending far upon the disk. Two of the median bars may 

 be joined at their lower ends. 



The generally subtrapezoidal shape of the shell, with flat sides, and the simple 

 character of the beak-sculpture are the chief characters of the group, which may 

 represent the most primitive type within the genus. The posterior end of the 

 shell is mostly not pointed, but more or less rounded, and the posterior ridge is 

 rather indistinct, not prominently angular. 



A great number of "species" belong here, which are extremely hard to dis- 

 tinguish. It is in this group that I encountered the greatest difficulties in the 

 identification of the species, and even at present I am not satisfied with the results. 

 This is the more to be regretted, since I have abundant material with soft parts of 

 some of these forms and have found that there are differences in the anatomy, 

 which to all appearance are of specific value. 



After many attempts to group these forms, I have finally concluded that the 

 best way, the one that is least hable to lead into error, is to treat these forms geo- 

 graphically. It is not very likely that the same identical species occurs in widely 

 separated river-systems. 



Forms of this type are found over nearly the whole continent, but apparently 

 they are rare or missing in the region of the great depression which runs from the 

 La Plata up the Paraguay to the Amazon-drainage. I shaU begin with the forms 

 from Chile and Patagonia, and proceed then in the direction toward Brazil, going 

 from South to North. 



1. Species from Chile and Patagonia. 



I have very insufficient material of this group, and cannot express any positive 

 opinion about the species. But it seems that all, or nearly aU, of the forms known 

 from Chile belong here. Simpson (1914, p. 1257) admits ten of them: chilensis 

 Gray, solidulus Philippi, gassiesi Kuester, aplatus Reeve, molince Philippi, modestus 



