260 NEW YORK STATE MUSEUM 
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 for- 
ward and having their visual surfaces so extended that the extremities 
of each edge approach each other [pl. 45, fig. rt and S. myops]. 
The relationship of the two is strikingly brought out in the form of the 
metastoma. The elongate subrectangular outline in D. macro- 
chirus is well shown in plate 45, figure 3. It obviously corres- 
ponds 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 
asthatof D. macrochirus. similar metastomas are quite common 
in the Otisville beds, where two species of Stylonurus and one of Doli- 
chopterus 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 dif- 
ferent 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. . : 
