CONCLUSIONS. 93 



pared with the range of Luxilus cornutns. In the genus Ceraticlithyn^ C. 

 hignttatus probably occurs in every stream from the Susquehanna to the 

 Great Salt Lake, while four other species of the same genus, C. micro- 

 pogon, C. monachiis, C. zanemns, and G. labrosus, are each, so far as is 

 known, coutined to a single river-basin. 



XII. In any river-basin, the most abundant species (of small fishes) 

 are usually (a) those i)ecnliar to it, or {b) those of the widest distribution. 

 In illustration of this, we may notice the abundance of Codoma jnjrrho- 

 melas and Noiropis photogenis in the Santee; of Codoma stigmatura and 

 IaixHus cornutns in the Alabama; of Codoma euri/stonia and Ceratichthys 

 hignttatus in the Chattahoochee; of Codoma xceniira and Notemigonus 

 americanus in the Ocmulgee. To this rale, however, there are mauy 

 exceptions and modifications. 



XIII. In general, the further south any 'river-basin lies, the more 

 species are peculiar to it, and the greater the differences between its 

 fauna and that of the neighboring streams. In illustration of this, the 

 differences existing between the faunj© of the Alabama and Chatta- 

 hoochee may be compared with those between the faunae of the Susque- 

 hanna and Delaware. Twelve genera are known to be common to the 

 Chattahoochee and Alabama, and twenty-three to the Susquehanna and 

 Delaware. In the- Southern streams, the process of evolution of specific 

 forms seems to have gone on more rapidly. This matter, however, 

 requires further investigation. 



XIV. Species of the widest distribution often have breaks in their 

 range which cannot be accounted for by any facts now in our posses- 

 sion. Luxilus cormitus, so abundant in all the waters of the North and 

 West, does not occur, so far as is known, in any of the rivers between 

 the Neuse and the Alabama, in both of which streams it is abundant. 

 A^arious species range over several river basins and then cease abruptly. 

 Amiurus hrunneus is abundant from the Santee to the Chattahoochee, in 

 the latter river the most abundant food-fish, while in the very next river- 

 basin, the Alabama, it is unknown. 



XV. Many species of wide distribution which are absent in certain 

 streams are there represented by certain other related species, which 

 may be regarded as modified descendants. Thus, in the South Atlan- 

 tic streams, Chcenohryttus gulosus is represented by Chcenobryttiis viridis, 

 Xotemigonus chrysoJeucus by Kotemigomis americanus. In the South- 

 west, Eupomotis aureus is represented by Eupomotis pallidus ; in the 

 West, Noturus gyrinus by Kottirus sialis, Noturus insignis by Noturiis 



