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 would at once have seriously interfered with the movements of the creatures in several ways; one, because the line of fusion would be directly under the important articulation between the carapace and preabdomen, and another, it could not have failed to disturb the mutual independence of two organs of entirely distinct character and rhythm of movement, namely, the coxae of the swimming legs which the metastoma closely adjoins, and the operculum with its respiratory and sexual functions.

The form of the metastoma has been found by us to be highly characteristic of the genera of the eurypterids and to be a good indicator of their phylogenetic relations. That of Eurypterus is typically oval in

outline, while those of Hughmilleria and Pterygotus show a strong tendency to become wider in the anterior half and narrower in the posterior, and at the same time more deeply emarginate in front. In Slimonia this tendency is carried to an extreme, the metastoma having become very elongate-cordate with a narrow posterior half and a deeply emarginate anterior one. In Dolichopterus and the Stylonurus branch in general, again a different tendency is developed. Here the base of the metastoma becomes rectangular, the lateral margins subparallel and the plate approaches a long rectangle, with the short front side deeply emarginate (Stylonurus) or it becomes lyrate (Dolichopterus). Finally, in Eusarcus