sure, although it remained monotypic until Holm referred two metastomas from the dolomitic shale of Rootziküll, Island of Oesel,[1] to this genus. The Bertie waterlime of Williamsville, near Buffalo, which has furnished the genotype, has afforded the carapace of a second species, here described as D. siluriceps, and the shale of the Shawangunk grit at Otisville has furnished a third species which sheds much interesting light on this rare and little known genus. We have named this last species D. stylonuroides because the carapace has all the characteristics of that of Stylonurus [pl. 46, fig. 13] in its subrectangular outline, broad doublure with frontal triangular plate on the underside and broad rim of the dorsal side, position, relative size and form of the lateral eyes. Its most peculiar features are, however, its swimming feet which appear beset with leaflike plates, the last segment being tripartite and consisting of three lanceolate plates. It is not difficult to recognize in its structure a further development of the peculiar lobelike lengthening of the segments of the last pair of legs noticed in D. macrochirus and to see in the tripartite extremity of the leg the result of the strong development of the ninth segment characteristic of Dolichopterus, the middle lobe representing the ninth, the lateral ones the preceding eighth segment. The same development of lobes is shown on the preceding pair of legs in the genotype and the leg retained in D. stylonuroides still further resembles that balancing leg as the last segment is developed into a spine, flanked by two lobes.
This interesting combination of a Stylonurus carapace and Dolichopterus leg in the same specimen suggests the question of the relationship of these two genera. A comparison of the two shows that Dolichopterus is more closely related to Stylonurus than to Eurypterus as a subgenus of which it was regarded by Hall and with which it has been associated by later authors. The similarity in the outlines of the carapace is manifest; they also have in common the distinctness and great width of rim,
- ↑ As we shall presently show, the metastomes of Stylonurus and Dolichopterus are alike or very similar and the two genera closely related. It is therefore possible that more complete material of the Oesel species will show them to belong to Stylonurus.
