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 great exterior similarity of the eurypterids and ostracophores and the explanation of this phenomenon as resulting from adaptation to like conditions. The ostracophores have been generally regarded by paleontologists as owing their peculiar form to their mud-grubbing habit, and it may be inferred that the eurypterids, being of similar form, were of like habit and perhaps of like form because of similar habit.

In this chapter we shall first survey the geological distribution of the eurypterids in North America as indicated by the following conspectus, compare this distribution with that in Europe, and finally attempt a conclusion as to the physical conditions under which these strange creatures lived.

Beltina danai Walcott. Greyson shales, Montana

Strabops thacheri Beecher. Potosi limestone, St François county, Missouri