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178 underside of the genotype would likely be of great interest and perhaps as surprising as the dorsal.

Basing the generic differences on the carapace, position of the eyes and type of opisthosoma, I recognize several genera which previously had been included in the genus Stylonurus. These are Brachyopterella, Parastylonurus, Stylonuroides and Stylonurella. Unfortunately, the genus Stylonurus remains with only one species, the genotype. Other fragmentary specimens are questionably assigned.

It is significant to recall that Clarke and Ruedemann, in their division of the stylonuroids in 1912, recognized the dilemma of the holotype and wrote, "Only a single specimen, not very favorably preserved in sandstone, has been recorded and although Page's not very correct figure and explanation have been followed by Woodward's elaborate description and careful illustration, this genotype is still incompletely known. Indeed, when the investigation of a considerable number of species revealed to us the presence of divisions of undoubted subgeneric rank, it remained doubtful with which of these the genotype belongs and which of the divisions therefore represents Stylonurus sensu stricto." (1912, p. 278). The divisions given here should help to alleviate this confusion, although problems still remain.

The species recognized for this genus are:

Diagnosis.—Stylonuridae of small size; carapace elongated-parabolic, longer than wide, surrounded by conspicuous wide marginal rim; eyes elongated-reniform, with axes parallel, and located so that their posterior borders are in the anterior half of the carapace; ocelli situated approximately midway between the eyes; third walking leg (IV) short, and with single opposable spines; fourth and fifth walking legs (V, VI) devoid of spines, except at the terminal end of the last leg which has either two flat spines or structures that might be interpreted as narrow supplementary lobes; podomere-count: ?–?–?–8–?–9.

Type species.—Stylonurus dolichopteroides Størmer, 1934.

Distribution.—Norway and New York.

Stratigraphic range.—Ordovician and Siluro-Devonian.