EIGENMANN : FRESH WATER FISHES. 239 



Order II. NEMATOGNATHI. 



Neniafognat/ii G\\\, Fishes of East Coast, 1870. 



The following opinions expressed several years ago in a then obscure 

 and now extinct journal are still, in part, pertinent and are repeated with a 

 few verbal changes. 



Not many fossils of this order have been found, and those which have 

 been discovered do not help us materially in determining the interrela- 

 tions of the higher groups. 



The date of the differentiation of the orders of the Ostariophysi is 

 uncertain. Silurinae, Bagarinae and Pimelodinae were separated as early 

 as the beginning of the Tertiary, and as these subfamilies are quite remote 

 from the most primitive living nematognathid, the Nematognathi must 

 extend into the Secondary period. 



The Nematognathi reach their highest development in the neo-tropics, 

 where they constitute forty per cent, of the entire freshwater fish fauna. 

 Most of the families, sub-families and genera now inhabiting this region 

 have undoubtedly originated here. In most of the families the maxillary 

 bone is a mere vestige and serves only as a support for a highly special- 

 ized dermal appendage, the maxillary barbel. So greatly has this been 

 modified, even in its development, that Ryder seriously questioned whether 

 the basal bone of the maxillary barbel in the North American cat-fishes 

 is in reality the vestige of a maxillary. No doubt need, however, be 

 entertained on that score, since in Diplomyste the basal bone of the short 

 primitive maxillary barbel is a functional dentiferous maxillary. 



The comparative development of the maxillary is, then, an excellent 

 guide to the determination of the rank any family is to occupy in the sys- 

 tem. 



Other structures valuable in this respect are the different barbels so 

 highly developed in some forms. Of the development of the barbels of 

 Ictalurus albidus, Ryder says : 



" The remarkably developed barbels of the embryos of the species make 

 their appearance very early, especially the maxillary pair ; these appear on 

 the second day. . . . The barbels on the lower jaw do not appear till the 

 fourth day of the development is completed. . . . The last of all to be 

 developed is the nasal pair, . . . (which) does not appear until the seventh 

 day." 



