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95 The Northern crayfish is found in most all of the state so it doesn't seem to be picky about the type of streams or landforms it inhabits. It is common in the two oldest watersheds, the Platte and Republican, so it is a good probability that these served as glacial refugia. The other watersheds are all younger than these so had to have been occupied later. The Glacial Till area dates from the end of the Kansan glacial period which ended some 500,000 years ago. At that time, the Platte and Republican were both flowing to the southeast into the Old Grand-Missouri watershed. If so, was this the route the crayfishes used to move into new watersheds? It would be interesting to conduct a detailed genetic study of this species. Would it tell us if the Big Blue and Loup basins were really connected at some distant time in the past? Would it give us a timeline as to when watersheds were occupied? Would it tell us how watersheds were connected? Calico crayfish, Orconectes immunis This map shows that the Calico crayfish, Orconcectes immunis, is widespread in the north-central U.S. As with the Northern crayfish, the blue line denoting the maximum extent of the ice shows that most of this range would have been under the ice during the Pleistocene. It also shows what other authors have suggested; that the refugia for this species was the western portion of the Missouri River basin. The map shows that they seem to prefer the glaciated portions of the upper Midwest. The southern limit of their range closely follows the southern limit of the glaciation and they are absent from the unglaciated region in southwest Wisconsin. In contrast to the Northern crayfish, the Calico crayfish is absent (or nearly so) from several ecoregions in the center part of the state. It most common in the eastern glaciated region as well as the western un- glaciated areas. It is virtually absent from the Republican basin and is present but uncommon in the Platte. Similarly, it is uncommon in the Loup, Elkhorn and lower Niobrara basins. The small pocket of collections in the central Sand Hills might represent a recent introduction as there are several fishing lakes in that area. The Platte River could have been a possible glacial refugia but the map suggests that it might have been further north also. The frequency of collection in the White River basin in the extreme northwest suggests this. This cannot be resolved here as there is a lack of crayfish distributional data out of South Dakota. The frequency of collection in the upper Niobrara suggests a geological connection between that basin and those to the north. If this is so, then their presence in the eastern glaciated region would argue that they came down from the northwest to occupy this area. Again, a detailed genetic study might give us some clues as to what happened.