Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/13

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Brachyopterus, Størmer, 1951
Brachyopterella, new genus
Clarkeipterus, new genus
Melbournopterus, Caster and Kjellesvig-Waering, 1953
(?) Dorfopterus, Kjellesvig-Waering, 1955

Genus Stylonurus Page, 1856, emended

Diagnosis.—Stylonuridae of medium size, prosoma subquadrate, lateral margins slightly rounded outward to reach greatest width at midsection, anterolateral angles rounded; eyes arcuate, located in posterior half of prosoma; podomere-count unknown, but the last two legs are greatly elongated, the last one extending to at least the twelfth tergite; opisthosoma wide, short, trilobated, and without differentiation into a pre- and post-abdomen; metasoma grades into the long telson without differentation; underside unknown.

Type species.—Stylonurus powriensis Page, 1856.

Distribution.—Scotland, Pennsylvania (?) and New York (?).

Stratigraphic range.—Devonian.

Remarks.—The above diagnosis is an emendation, but based only on the type species. However, it is necessary because Stylonurus, as was the case with the genus Eurypterus (Kjellesvig-Waering, 1948, p. 4; 1958, p. 1107), has become a convenient entity to which to refer practically any eurypterid with stylonuroid legs. Species have been referred to this genus on the basis of characters conforming to those of the type species in only the most general way, serving to place them in the superfamily Stylonuracea. This criticism is not aimed at any particular worker in the eurypterids, and it includes myself as much as anyone else.

Page, in his original description (1856, p. 135), described the genotype as "quaint-looking," and indeed it is, remaining as unique and enigmatic today as it did over one hundred years ago. The characters distinguishing Stylonurus from other stylonurids are not only the position of the eyes in the rear of the carapace, and the shape of the carapace, but more importantly the distinct trilobation, not only of the mesosoma as in some other eurypterids (Megalograptus, Mixopterus) , but also of the metasoma, a feature not found elsewhere except in the recently described Pagea of Waterston (1962), and the Woodwardopteridae, both considerably different eurypterids. Even without the presence of trilobation, the short, wide opisthosoma, without the slightest trace of differentiation, is a significant feature. The