Eurypterida
Eurypterids are rarely encountered in the Mazon Creek area, although the number of known specimens has increased gradually since the first report of the holotype of Adelophthalmus mazonensis by Meek and Worthen in 1868. In 1948 (p. 17), Kjellesvig-Waering reported on seven more specimens. Up to the present, however, only these eight specimens have been reported in the literature, all of them representing one species, Adelophthalmus mazonensis (Meek and Worthen).
The purpose of this notice is 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.
The complete list of Mazon Creek eurypterids is as follows:
| Adelophthalmus mazonensis (Meek and Worthen) | Mazonipterus cyclophthalmus, new gen. and sp. |
| Mycterops, sp. indet. | |
Acknowledgments.—I am particularly indebted to Dr. Eugene S. Richardson, Jr., as solely through his co-operation I have been able to locate the specimens of Mazon Creek eurypterids, which are in the hands of private collectors residing principally in the Chicago area. It is mainly through the ceaseless efforts of these avid collectors that the bulk of the Mazon Creek fauna has been revealed to science. I am also indebted to Dr. Willard P. Leutze, who told me about the important specimen PE 6263, which was at that time in the private collection of Mr. Bruce Bell, of Flossmoor, Illinois; Mr. Bell has since presented it to Chicago Natural History Museum. Mr. Jerry Herdina, whose remarkable collection of Mazon Creek eurypterids was kindly lent for description, particularly deserves acknowledgment. Acknowledgments are also due to Messrs. James
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