Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/20

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Diagnosis.—Medium-sized stylonuroid eurypterid; shape of prosoma probably subrectangular, lateral eyes arcuate, in the anterior half of the prosoma, subcentral; anterior margin of prosoma straight; third and fourth prosomal appendages bearing a double row of numerous flat spines, fifth and sixth prosomal appendages simple, keeled, tapering, sixth appendage reaching as far as the pretelson; podomere-count: ?–?–?–7–9–10; abdomen unusually slender with lateral epimeres on the postabdominal segments, axial furrows (trilobation) present on all abdominal segments; telson long, styliform, keeled. (Modified from Waterston, 1962, pp. 937–938.)

Type species.—Pagea sturrocki Waterston, 1962.

Distribution.—Scotland and Wales.

Stratigraphic range.—Devonian, Lower Old Red Sandstone.

Remarks.—This remarkable and unique eurypterid differs so widely from all others that comparison is superfluous. The unusually narrow opisthosoma, with trilobation, is unknown in any of the other genera of the Stylonuracea. It appears likely that trilobation in a narrow form such as this may be connected with strengthening the presumably highly flexible opisthosoma. However, this is not true of the trilobation in Stylonurus powriensis Page, a species which is noteworthy for its wide and heavily constructed opisthosoma. The podomere-count, ?–?–?–7–9–10, is of considerable generic importance in my opinion, and it differs from those of other Stylonuracea.

Two species are recognized for the genus:

Pagea symondsii (Salter) reveals the same shape of carapace, with eyes located at the same position, as well as having the wide anterior marginal rim. Although the holotype is a dorsal impression, both Salter (in Huxley and Salter, 1859, pl. X, fig. 1) and Woodward (1866–78, pl. XXI, fig. 4) show two diagonal depressions which very likely are the reflection of the epistomal sutures; thus P. symondsii (Salter) must have had the same wide epistoma that Waterston found in P. sturrocki.

Diagnosis.—Stylonuracea with metastoma ovoid, its anterior cordate, and posterior rounded; ventral shield unknown.