The question of the phylogenetic relation of Dolichopterus to Stylonurus, Drepanopterus and Eurypterus has been fully dealt with in the introductory chapter to which we here refer.
Besides the three species of Dolichopterus from the New York rocks thus far mentioned, the Bertie waterlime at Litchfield and the Shawangunk grit at Otisville have each furnished a type represented only by the carapace; D. testudineus and D. otisius; and the Frankfort shale has afforded the fragmentary remains designated D. frankfortensis and D. latifrons.
From the waterlime beds of Litchfield we have a swimming leg, which possesses in general the characters of Dolichopterus but has a greatly differing palette or terminal segment. The latter is elongate oval and strikingly resembles the palette of Pterygotus. As the palette in Pterygotus is the eighth segment, while that of Dolichopterus is the ninth, this similarity can hardly be more than the result of convergence. This limb is as far different from that of the genotype, D. macrochirus, in one direction, as that of D. stylonuroides is in another. In this the relative compactness and strength of the swimming leg of D. macrochirus is carried to extreme, in the other the tendency of D. macrochirus to the development of broad lobelike appendages.
These few species together indicate a greater variability in the characters of this genus than is shown by other genera.
The genus is at present represented by seven American and two European species, viz:
| D. macrochirus Hall | D. stylonuroides nov. |
| D. frankfortensis nov. | D. testudineus nov. |
| D. latifrons nov. | D. laticeps (Schmidt) |
| D. siluriceps nov. | D. sp. Holm[1] |
| D. otisius Clarke |
- ↑ As before stated, Holm recognized the probable occurrence of this genus in the Baltic provinces by the observation of two metastomes, one of which he has referred to Eurypterus laticeps Schmidt. This species is known only by its carapace which in outline, broad rim, large eyes and ornamentation is very suggestive of the Stylonurus-Dolichopterus group. He also referred an operculum with female
