452 MEMom.s OF the carnegie museum. 



The present paper is intended to give a full account of these investigations. 

 Naturally all accessible material has been included. In many cases only shells 

 without their soft parts were available. This remark refers largely to older and 

 some more recent material preserved in the Carnegie Museum obtained from various 

 sources. The Haseman Collection likewise contains only the hard parts of many 

 species, but on the other hand, other accessions consisted of shells with soft parts, 

 as, for instance, the specimens of Anodontites crispatus from the upper Magdalena- 

 drainage in Colombia, collected by Dr. C. H. Eigenmann, and a collection of a 

 number of species from the La Plata in Argentina, received from Dr. A. Wind- 

 hausen. 



In si^ite of the comparative wealth of material at hand, in some respects our 

 collections are not complete and do not permit the determination of certain ques- 

 tions, chiefly taxonomic. My greatest difficulty has been to properly identify 

 the "species" at hand. The older writers, Spix, Wagner, Lea, D'Orbigny, Philippi, 

 Hupe, and others, generally described their "species" from very insufficient ma- 

 terial, and the author to whom we are most indebted for clearing up the taxonomy 

 of the South American Naiades, H. Von Ihering, also was often handicapped by 

 having too scanty material. All previous writers had no clear conception of the 

 range of variation in the various forms, and thus their descriptions generally are 

 those of individuals, often indeed very elaborate and complete, but without proper 

 emphasis laid upon the really important specific characters. Simpson's "Descrip- 

 tive Catalogue" (1914) did not much improve matters, since he was largely de- 

 pendent upon the unsatisfactory publications of previous authors. 



I do not claim, by any means, that my treatment of these shells overcomes all 

 these difficulties; on the contrary, I am in many particulars not at all satisfied 

 with the results obtained. Nevertheless, I claim that the study of the soft parts 

 of a great many forms has furnished a basis for the proper understanding of what 

 the "species" are, and has, at least, furnished a clue to their systematic arrange- 

 ment, incomplete and fragmentary, it may be, but which probably will prove to 

 be of great value, when the soft parts of all or most of the species are known. 



The difficulty encountered in recognizing the described forms is sometimes 

 exasperating. We should expect, in cases where exact type-localities are given, 

 and where material from these is at hand, that it would not be very great. But, 

 for instance, even of the species described by Von Ihering from Sao Paulo, I have 

 recognized only a comparatively small number, although I possess a large quantity 

 of material representing the Naiades from that stat(\ In other cases, when the 

 type-locality is vaguely, or not at all, given, it has been practically impossible to 



