Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/17



Diagnosis.—Stylonuridae of small size; prosoma with greatest width anteriorly, length equivalent to greatest width, base narrower than anterior, rounded anteriorly; eyes elongated-reniform, with axes slightly converging; first leg developed as grasping organ, all other legs very stout, with few spines, stylonuroid but not excessively developed; opisthosoma short, relatively broad, undifferentiated; metasoma tapering into clavate, spike-like telson. (Modified from Størmer, 1951, p. 416.)

Type species.—Brachyopterus stubblefieldi Størmer, 1951.

Distribution.—Wales.

Stratigraphic range.—Ordovician.

Remarks.—There is no other genus known that warrants comparison, or, indeed at our present state of knowledge, reveals any close affinities with this very unusual genus; but see Remarks following the discussion of Brachyopterella.

Diagnosis.—Stylonuridae of small size; carapace pentagonal, with little or no marginal rim and with concave base; eyes oblique, close-set, placed anteriorly on the carapace, very long, banana-shaped, and with axes converging and crossing a short distance in front of the eyes; ocelli placed well forward between the eyes; epistoma well developed; legs increasing gradually, the first three with short opposable spines at the end of each joint, the last two walking legs longer but without spines; podomere-count: 3?–7–8–8–9–?; metastoma unknown; ornamentation smooth on carapace, legs partly covered with granulations.

Type species.—Stylonurus pentagonalis Størmer, 1934.

Distribution.—Norway.

Stratigraphic range.—Ordovician (?), Siluro-Devonian.

Remarks.—The genotype has been included in the Middle Ordovician genus Brachyopterus. However, the differences are much too great to include the Norwegian form with the latter. The shape of the carapace, and the altogether different type of eyes, as well as their