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.

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