| Subgenus D. (Drepanopterus) | |
| S. lobatus Laurie | S. bembycoides Laurie |
| S. pentlandicus Laurie | S. longicaudatus nov. |
| Indeterminate species | |
| S. megalops Salter | S. beecheri Hall |
| S. ensiformis H. Woodward | S. ? limbatus nov. |
| S. symondsi (Salter) | S. myops Clarke |
Among the American species only group B (Ctenopterus) and group D (Drepanopterus) have thus far been recognized, the former with one, the latter with two species. The subgeneric relations of the four other American species: S. excelsior, S. beecheri, S.? limbatus and S. myops remain undetermined.
Stylonurus (Ctenopterus) cestrotus Clarke
Eurypterus? cestrotus Clarke. N. Y. State Mus. Bul. 107. 1907. p. 307, pl. 3, fig. 8–10
The original description of this species reads: "Of this species we have only enough to satisfactorily establish its difference from other forms—the two specimens here illustrated. Both show the peculiarly ornamented frontal border of the cephalon which carries a row of denticulations. One of these specimens conveys a satisfactory idea of the form of the body, and presents the ventral aspect but there is some uncertainty in regard to the number of segments and though evidences of four pairs of legs are present the structure of these is not apparent. The head shown in figure 10 indicates that the compound eyes are large and very far forward. It is entirely probable that when this species becomes better known it will have to be excluded from the genus Eurypterus."
The larger collections acquired since this date and the development of the counterpart of the first of the two specimens figured in the preliminary paper, have afforded sufficient data to form a fairly accurate conception of this most peculiar type.
Description. Body of small size, slender and terete, all parts being noticeably elongate.
