Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/16

180 of Stylonurella differs greatly from the subrounded, wider than long, carapace of Parastylonurus.

In the shape of the carapace and position of the eyes, Stylonurella has a resemblance to Kokomopterus, however the altogether different type of prosomal appendages is a difference that precludes the necessity for further comparison.

Recognized species for this genus are:

Diagnosis.—Stylonuridae of small to medium size; prosoma rounded-quadrate; wider than long, with lateral eyes, arcuate and located on anterior half of the carapace; metastoma very long, with deep triangular notch or deeply cordate and with base truncated; first three walking legs very short with single opposable spines on each joint; last two walking legs very long, reaching to the last tergite; podomere-count: ?–?–7?–8–9–9; ornamentation consists of scales; opisthosoma narrow, undifferentiated; telson spike-like.

Type species.—Stylonurus ornatus Laurie, 1892.

Distribution.—Scotland, England, New York and Pennsylvania.

Stratigraphic range.—Ordovician, Silurian and Devonian.

Remarks.—Parastylonurus includes more forms previously included in the genus Stylonurus than do the other genera described here. However, in my opinion, the development of distinct trilobation in the genus Stylonurus sets that genus irrevocably apart from the species that are here referred to Parastylonurus. The position of the eyes is also a major generic difference.

The following species are recognized: