Page:The Habitat of the Eurypterida.djvu/215

Rh and it is possible that they might have been able to withstand exposure to the air for several hours. In passing from one stream to another their locomotion would be fairly rapid and their migration in this manner might not have been infrequent.

By the discriminating use of the laws which have been observed to be potent in directing the migration of fishes and other organisms living in the rivers at present, and without making any unwarranted assumptions, it seems safe to postulate the following expectabilities in regard to the geological and geographical distribution which we should be able to find among the eurypterids, providing they lived in the rivers.

1. Unless, as some have supposed, but which is very improbable, there were no climatic zones in the Palaeozoic, and conditions of temperature were equable over the whole globe, related or identical species of eurypterids should be found in deposits geographically situated in a circumpolar zone, not necessarily the same as the climatic zones of the present.

2. Eurypterid remains should be expected to occur in deposits of limited areal extent marking lake sediments, flood plain deposits, or littoral deposits in the sea at or near the mouths of rivers.

3. Eurypterids which inhabited the streams of one river system would be more closely related than those living in the tributaries of different and entirely distinct systems, and in general this would mean that forms which lived in the rivers of one continent in any period, would constitute a group of related genera and species, while those living in the rivers of another continent would constitute a distinct group, the individuals of which would be related; and if the different continents should remain unconnected for a long time, geologically, distribution and evolution would continue on each land mass, but we would not expect any of the individuals from one continent to migrate to another, so that succeeding faunas should not show intercontinental affinities, though phylogenetic relations should be discernable on each continent. It must be remembered, however, that remains of faunas from both continents might be carried into basins which received the simultaneous drainage of rivers from each.