Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/25

Fig. 51. The only known, well-preserved, and entire male (Type A) mesial appendage of Adelophthalmus. This occurs in a specimen of A. mansfieldi (C. E. Hall) from the Pennsylvanian, Allegheny group at Cannelton, Darlington township, Beaver County, Pennsylvania; previously figured by James Hall (1884).

In Adelophthalmus mansfieldi and A. mazonensis the length/width-at-base (of the carapace) ratios and the length/width-behind-eyes ratios show little difference when plotted and when not taking into consideration the important preservational factor. For instance, all of the carapaces of A. mansfieldi are preserved in black shale and almost without exception they are completely compressed into one plane. Most of the carapaces of A. mazonensis, on the other hand, are only partly compressed and retain, in many cases, considerable relief. Thus A. mansfieldi is a much narrower form than A. mazonensis, as compressed carapaces of A. mansfieldi have nearly