In contrast to the Silurian, the North American Devonian deposits have not yielded a rich fauna of eurypterids, although the list of known species is continuously growing. The occurrence of eurypterids in the Holland Quarry shale, a lens locally underlying the Sylvania sandstone at the base of the Devonian in Lucas County, Ohio (see Carman, 1960), adds materially to our relatively meager knowledge of Devonian forms in North America.

This is a scientific study of a geological nature carried out on the subject of Eurypterids (extinct sea scorpions). The study report aims to record new morphological and biometric data on twenty-three hitherto unreported specimens of A. mazonensis (Meek and Worthen), and to describe two new eurypterids previously unknown in the Mazon Creek fauna. One of these, a very unusual eurypterid of the family Stylonuridae, is described as a new species of the new genus Mazonipterus, and the other is represented by fragments of a specimen belonging to Mycterops, a peculiar genus previously reported in Pennsylvanian beds of Pennsylvania, Belgium and Holland.

1919/28 cumulation includes material previously issued in the 1919/20-1935/36 issues and also material not published separately for 1927/28. 1929/39 cumulation includes material previously issued in the 1929/30-1935/36 issues and also material for 1937-39 not published separately.