while the doublure in D. macrochirus is much narrower than in the typical Stylonurus. The eyes differ in both genera from those of Eurypterus in being relatively much larger, placed as a rule further forward and having their visual surfaces so extended that the extremities of each edge approach each other [pl. 45, fig. 1 and S. myops]. The relationship of the two is strikingly brought out in the form of the metastoma. The elongate subrectangular outline in D. macrochirus is well shown in plate 45, figure 3. It obviously corresponds in length to the much elongated carapace and the long coxa of the swimming leg. The two metastomas figured by Holm are of like shape. Laurie has succeeded in tracing the outlines of the metastomas of two species of Stylonurus [text fig. 64] and they exhibit the same form as that of D. macrochirus. Similar metastomas are quite common in the Otisville beds, where two species of Stylonurus and one of Dolichopterus occur. The form of this organ appears to furnish an excellent criterion of generic unity, as here demonstrated among the species of Eusarcus and Pterygotus, and it is inferred that like metastomas in different genera are strong evidence of close relationship.
Further, we have here shown that at least one subgenus of Stylonurus, viz, Ctenopterus, is characterized by long, straight, relatively thin, closely arranged spines. The same feature is repeated in D. macrochirus [see drawing of type, Hall, pl. 83A, fig. 1 and, here, pl. 45, fig. 3].
This similarity of the legs continues in the last two pairs, and shows itself in the great lengths of the fourth and fifth segments.
Finally the slender, gradually tapering abdomen of Dolichopterus with its winglike epimera [pl. 43] is also more suggestive of Stylonurus than of Eurypterus or any other of the genera of the eurypterids. We suspect that the telson will prove to be styliform in Dolichopterus as in Stylonurus.
The main difference between Dolichopterus and Stylonurus is in the development of the distal parts of the last pair of legs; in Dolichopterus these are broadened and beset with leaflike lobes, while in Stylonurus they are narrow and greatly lengthened. The preceding pair of legs is also furnished with lobelike appendages in Dolichopterus and is free of such in Stylonurus.
