Page:The Habitat of the Eurypterida.djvu/228

220 of a Bertie species. The evidence is frail, and yet it might seem a little disconcerting to have an individual which came from Appalachia, as we think, showing relationship to one coming from Atlantica, but I shall have a suggestion to make when I come to the Bertie that will do away with even this slight difficulty, which is, after all, entirely negligible, since the only specimen showing relation to a Bertie form is a single metastoma which bears only a suggestion of similarity. Continuing in chronological sequence, the next formation in which a Dolichopterus occurs is the Shawangunk grit where there are two species D. otisius with a large representation of carapaces none of which retain more than two body segments, and D. stylonuroides of which three carapaces and one more complete individual have been found. The former species has certain characters in common with D. macrochirus (Bertie), the young of both being even more alike than the adults. If the two species are phylogenetically related, then the adult D. macrochirus has kept the ancestral characteristic of a broad frontal lobe on the carapace, for this is found in the young of both species and retained throughout the ontogeny of the Bertie form, while D. otisius in the adult shows a development of this lobe into an angular extension. In this one characteristic, then, D. macrochirus would show retardation. The second species in the Shawangunk is rare and shows no close relationship to any known species.

In considering both the Schenectady and Shawangunk faunas, we have seen that there was a species of Dolichopterus in the latter and a single specimen in the former which showed a more or less close relationship to certain species of the same genus in the Bertie. From purely stratigraphic reasoning it is known that specimens of the first two formations were derived from Appalachia, while those of the Bertie came from Atlantica. The question might be raised whether the stratigraphic facts do not conflict with my biological theories, for I have been trying to show that eurypterids found in sediments which were transported by rivers on the same continent should show genetic relationship and should for the most part be distinct from those which lived in rivers on different continents; but the species of Dolichopterus do not seem to conform to this law. In the case of this particular genus with its distribution in time, there would be no apparent physical objection to the accounting for its affinities on the very simple assumption that the Cambric or earlier generic ancestors lived in rivers either on Appalachia or on Atlantica and that during one of the peri ods when these continents were connected by a strip of land the