Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/15

 Remarks.—Stylonuroides can easily be distinguished from all other stylonurids by the very long carapace and reniform eyes, as well as the details of the termination of the last walking leg. It is, as the species name of the genotype indicates, and as Størmer (1934, p. 103) has pointed out, another link between the Dolichopteridae and the Stylonuridae, a further step in the transition from species such as Ruedemannipterus stylonuroides (Clarke and Ruedemann). However, the long carapace and reniform eyes, a considerable departure from the usual greatly arcuate eyes of the stylonurids, may well reveal some affinities with the family Rhenopteridae.

The following species are recognized for the genus:

Diagnosis.—Stylonuridae of small size; subquadrate prosoma, approximately as long as wide, slightly constricted at midsection; eyes parallel, arcuate, anteriorly located, intramarginally, about midway in the anterior half of the carapace; ventral shield with epistoma, but metastoma unknown; first two appendages also unknown, third and fourth prosomal legs very short, last two walking legs very long; podomere-count: ?–?–7?–8–9–9?; metasoma very narrow, non-trilobed, tapering.

Type species.—Stylonurus spinipes Page, 1859 (=Stylonurus logani Woodward).

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

Stratigraphic range.—Ordovician, Silurian and Devonian.

Remarks.—Although this was one of the earliest described stylonuroids, made known not long after the description of the genotype of Stylonurus, it bears no close (generic) affinities to that genus. The differences between this genus and Stylonurus are numerous and outstanding. Stylonurus has eyes placed on the posterior half of the carapace whereas Stylonurella has them anteriorly located. Perhaps of even greater importance is the wide opisthosoma with trilobation of stylonurus as against the narrow terete mesosoma without any trace of trilobation in Stylonurella. The differences of Parastylonurus also are numerous and obvious. The quadrate form of the carapace