NORTH AMERICAN RIVER FAUNAE. 521 



main river course. In the Mississippi, how- 

 ever, the case is otherwise and very striking, 

 inasmuch as we find here, at separate lati- 

 tudes, distinct species of the same genera, 

 somewhat like the differences observed in dis- 

 tinct water-basins ; and yet the river is ever 

 flowing on past these animals, which remain, 

 as it were, spell -bound to the regions most 

 genial to them. The question at once arises, 

 do our smaller rivers present similar differ- 

 ences ? I have already taken steps to obtain 

 complete collections of fishes, shells, and cray- 

 fishes from various stations on the Connecti- 

 cut and the Hudson, and their tributaries ; 

 and I should be very happy if I could include 

 the Susquehanna, Delaware, and Ohio in my 

 comparisons. My object in writing now is to 

 inquire whether you could assist me in mak- 

 ing separate collections, as complete as pos- 

 sible, of all these animals from the north and 

 west branches of the Susquehanna, from the 

 main river either at Harrisburg or Colurribia, 

 and from the Juniata, also from the Schuyl- 

 kill, Lehigh, and Delaware, and from the Al- 

 leghany and Monongahela. I have Swiss 

 friends in the State of New York who have 

 promised me to collect the fishes from the 

 head-waters of the Delaware and Susque- 



