The fishes of the Black Hills consist of two catfishes, four suckers, eight minnows, and one member of the cod family. Why are there no spiny-rayed fishes? If you examine a map you will find that the Black Hills is an isolated region, about seventy-five by one hundred miles in extent. It is covered with heavy pine forests and drained by a dozen or more good-sized creeks, which find, through the north and south forks of the Cheyenne, an outlet into the Missouri river. Surrounding the Black Hills is a broad plain one hundred or two hundred miles in width. It has no forests, and only a scant vegetation. Its streams are alkali and contain much solid matter in suspension. None of these streams flow over rocky or gravelly beds. Like all the streams of the great plains they are overloaded with sediment. All the streams can do with this sediment is to deposit it in places during falling or low water, and in time of freshets, pick it up, shift it about and redeposit it farther down the stream. Such streams are like the Platte, narrow and deep in a few places, but mostly wide and shallow, with a bottom of quicksand. The streams of the plains have in them but few species of fishes; especially is this true of the upper Missouri, and these are such species as we find in the Black Hills. It is thus evident that the fishes of this region migrated there, and only such fishes as were able or willing to live in the muddy, alkaline streams of the great plains could have ever reached the Black Hills. The minnows and suckers are ever preyed upon by sunfishes, bass and the like, and to escape them evidently sought retreat in the alkaline water, which was too much disliked by their enemies for them to follow. Once there and accustomed to such water they would migrate farther up stream until they reached the clear, cool streams of the Black Hills. If we compare the fishes of two rivers whose mouths are near each other, as the Ohio and the Missouri, those fishes found near the mouths will be the same species and the two river faunas will differ most as you go toward their sources. On the other hand, if you select two rivers whose sources are near each other, as the James and tributaries of the Ohio, then the fish faunas will differ most as you go towards their mouths. The same is true of the Missouri and the Columbia. In such cases it often happens that during high water some fishes are able to pass from the head waters of one river basin to the other, just as we see the trout from the Columbia at the present time colonizing the upper Yellowstone through the Two Ocean Pass. Near the head waters of many mountain streams there is usually a pass, which contains a strip of meadow land where the small streams from mountains unite, forming the sources of two great rivers flowing in opposite directions. This is the case both at the Two Ocean Pass, the source of the Missouri and the Columbia, and at the point where the Canadian