Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/30

 carapace, in dorsal aspect in a typical ironstone concretion. The specimen is partly compressed, although there is no appreciable distortion. What appears as a transverse joint-line across the middle

of the carapace is a narrow plant stem that lies on the outside of the carapace. This stem was excavated satisfactorily, and there is no doubt that it represents extraneous material and not the junction of the carapace with a long first tergite, as in the Scottish Woodwardopterus scabrosus (Woodward). A similar stem cuts diagonally across the carapace (see fig. 52).

The carapace is very long, surrounded by a very thin, unornamented, marginal rim, and is swollen at approximately midsection. It is therefore campanulate but narrowing toward the genal angles. The large, highly arcuate eyes, with disc-like palpebral lobes, are located on the anterior of the carapace, close together, and intramarginally. There is no trace of any ocelli or ocellar mound and enough of the carapace is present to assure their preservation if they were present. The surface is smooth.

At the anterior of the carapace, the ventral shield is faintly preserved as an impression reflected through the carapace. The holotype previously had been carelessly excavated with a sharp instrument toward the anterior, but part of the doublure can still be discerned. It appears to be strongly cordate (see fig. 52) as in Limulus or stylonurids such as ''Brachyopterus? pentagonalis'' (Størmer).