CONCLUSIONS. U3 



red with the range of Luxilus cornutus. In the genus Ceratichthys, C. 

 ttatus probably occurs in every stream from the Susquehanua to the 

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

 pogon, C. monachiiSy C. zanemus, and C. labrosus, are each, so far as is 

 known, contined to a single river-basin. 



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

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



illustration of this, we may notice the abundance of Codoma pyrrho- 



las and Nolropis pliotogenis in the Santee; of Codoma stigmatura and 

 Luxilus cornutus in the Alabama; of Codoma eurystoma and Ceratichthys 

 biguttatus in the Chattahoochee; of Codoma xcenura and Notemigonus 

 amcricanus in the Ocmulgee. To this rale, however, there are many 



ptious and modifications. 

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

 ies 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 faunaB of the Alabama and Chatta- 

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

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

 Cliattahoochee and Alabama, and twenty-three to the Susquehannaand 

 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 iu our posses- 

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

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

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

 Various species range over several river basins and then cease abruptly. 

 Am turns brunneus 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- 



-treams, Chwnobryttus gulosus is represented by Chcenobryttus viridis, 

 Notemigonus clirysoleucus by Notemigonus americanus. In the South- 

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

 West, Noturus gyrinus by Noturus sialis, Noturus insignia by Notunts 



