Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/28

 least dorsally, and that with more material several may be found to be conspecific, and certainly several must be delegated to sub-specific rank (Kjellesvig-Waering, 1948, p. 5). It should be noted, however, that the eurypterids identified by Van Oyen from the "Veine D" as A. imhofi (Reuss) differ as much from the holotype of A. imhofi as from A. mansfieldi. The species from the "Veine D," in my opinion, is Adelophthalmus cambieri (Pruvost).

Diagnosis.—Stylonuridae of medium size; carapace very elongated, with lateral eyes arcuate and placed anteriorly on the carapace. The greatest width of carapace occurs midway. Palpebral lobe attached to carapace by a narrow bridge on outer-posterior part of lobe. Marginal rim very narrow, simple, not ornamented. Ornamentation smooth. No other parts known.

Occurrence.—Middle Pennsylvanian of Illinois.

Type species.—Mazonipterus cyclophthalmus Kjellesvig-Waering.

Remarks.—This is an easily recognizable and very unusual genus. The remarkably long, inflated carapace ind the large disc-like, arcuate eyes, well forward of the center of the carapace, recall the Silurian genus Ctenopterus Clarke and Ruedemann, 1912, and indeed, may be the Pennsylvanian straggler of that line. We know of no closely related forms between the Middle Silurian and the Middle Pennsylvanian, however, that might give substance to that supposition. The two genera differ in many important structures. Mazonipterus has a longer, more inflated carapace; an unornamented marginal rim; highly arcuate eyes, which are covered by disc-like palpebral lobes that join the carapace at the outer posterior part of the eyes; and no surface ornamentation. Ctenopterus has a much shorter, converging carapace; an ornamented marginal rim; sub-reniform, lateral eyes located on mounds; and prominent scale-like ornamentation. The Silurian Stylonurus dolichopteroides Størmer also bears some resemblance, which, however, might be of more importance when more is known of each. The shape of the carapace, as well as the shape of the lateral eyes, will readily distinguish both.

Mazonipterus is a highly unusual form, not only with regard to morphology but because it is the first definite stylonurid to be found in the upper Carboniferous. Augusta and Přibyl (1951, pp. 2–4,