CAMBRIDGE, July 9, 1853. . . .While ascending the great Mississippi last spring I was struck with the remarkable fact that the fishes differ essentially in the different parts of that long water-course,--a fact I had already noticed in the Rhine, Rhone, and Danube, though there the difference arises chiefly from the occurrence, in the higher Alpine regions, of representatives of the trout family which are not found in the main river course. In the Mississippi, however, the case is otherwise and very striking, inasmuch as we find here, at separate latitudes, distinct species of the same genera, somewhat like the differences observed in distinct 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 differences? I have already taken steps to obtain complete collections of fishes, shells, and crayfishes from various stations on the Connecticut 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 making separate collections, as complete as possible, of all these animals from the north and west branches of the Susquehanna, from the main river either at Harrisburg or Columbia, and from the Juniata, also from the Schuylkill, Lehigh, and Delaware, and from the Allegheny 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 Susquehanna within the limits of the State of New York. I cannot, of course, expect you to survey your State for me, but among your acquaintance in various parts of your State are there not those who, with proper directions, could do the work for me? I would, of course, gladly repay all their expenses. The subject seems to me so important as to justify any effort in that direction. Little may be added to the knowledge of the fishes themselves, for I suppose most of the species have been described either by De Kay, Kirtland, or Storer; but a careful study of their special geographical distribution may furnish results as important to zoology as the knowledge of the species themselves. If you cannot write yourself, will you give me the names of such persons as might be persuaded to aid in the matter. I know from your own observations in former times that you have already collected similar facts for the Unios, so that you will at once understand and appreciate my object. . .