alothorax and the nearly centrally located eyes, Syntomopterus resembles some of the species included under Erieopterus in the family Eurypteridae. However, the arcuate eyes, with the wide palpebral lobe, located behind the middle of the cephalothorax, as well as the anterior position of the ocellar mound, the flat, wide marginal rim, the narrow, unjointed ventral shield and the pattern of the ornamentation, which is much like that of Stylonurus megalops (Salter), precludes its inclusion in that family and aligns it in the family Stylonuridae.
The wide, paraboloidal cephalothorax is in marked contrast to other Stylonuridae. However, the cephalothorax of Stylonurus myops Clarke, of the Silurian Shawangunk sandstone of New York, reveals similar widening, although the form is different from that of Syntomopterus. The New York Silurian Erieopterus pustulosus (Hall) has much in common with this form. In all references (Hall, 1859, p. 413; Pohlman, 1882, p. 41; and Clarke and Ruedemann, 1912, p. 201) the authors have described and figured the compound eyes as being the Eurypterus type of reniform eyes and not the arcuate type found in Syntomopterus. It is possible that the palpebral lobe has been broken out in the specimens described, though there is no reason to believe that those authors would not have noted that easily recognizable condition. The eyes of E. pustulosus are placed in front of midsection, whereas those of Syntomopterus are behind. The ornamentation of E. pustulosus is in some respects strikingly like that of Syntomopterus although it differs greatly in the presence of pointed scales; it also lacks the wide, flat, marginal rim of the latter—an important difference.
I recently studied the types of the British Downtonian Stylonurus megalops (Salter), and it was mainly on that basis that I arrived at the conclusion that the Ohio form was probably a stylonurid. The compound eyes are somewhat similar in having the wide palpebral lobes, but, above that, the ornamentation on the cephalothorax and on the wide marginal rim is of the same type, although not as pronounced or as densely distributed as in Syntomopterus richardsoni. The similarity, however, ends with the ornamentation, as the form of the cephalothorax and the position of the eyes are entirely different.
Few eurypterid genera have the compound eyes situated on the posterior half of the cephalothorax. Some that have are the Australian Silurian Melbournopterus and the Scottish Devonian Tarsopterella, both of which are included in the Stylonuridae.
