INTRODUCTORY. 



In this paper we have endeavored to give keys to the 

 genera and species of Nematognathi ascribed to South 

 America, to collect the synonymy and bibliography in 

 full to date, and to give descriptions of the forms we 

 have been able to examine. We have thought best to 

 give full descriptions of most of the species in the 

 Museum of Comparative Zoology for several reasons. 

 Many of the forms are rare and contained only in a 

 single museum; and full descriptions of those species 

 in this museum will aid in identifications and revisions 

 of this group by ichthyologists working in other muse- 

 ums. As yet scarcely more than the main streams of 

 South America have been explored, and many new forms 

 will doubtless appear when the smaller streams are 

 searched; in that case detailed descriptions of as many 

 as possible of the known species in some one work will 

 be very desirable, especially since many descriptions 

 are now to be found only in inaccessible journals. 

 Furthermore, short diagnoses, although they may be 

 excellent and quite sufficient at the time they are writ- 

 ten, sometimes become worthless by the discovery of 

 new species which agree with the old in all the charac- 

 ters given in the diagnosis, but differ from them in 

 characters not mentioned. 



We are fully in accord with the statement recently 

 made by Dr. Cope that "descriptive zoology will never 

 be complete until the structure is exhausted in furnish- 

 ing definitions," and our only excuse for not treating in 

 detail of the internal structure, especially the osteology of 

 the siluroids, was the inability to mutilate series of 



