Associated with dismembered parts of Dikellocephalus minnesotensis in the St. Lawrence limestone, at Lansing, Iowa, are parts of a very interesting Eurypterid. The carapace is sub-semicircular in outline; anterior margin well rounded; posterior margin broadly concave; sides diverging gently posteriorly and somewhat produced at the postero-lateral angles. The compound eyes are prominent, bean-shaped, situated about in the middle, and as far apart as their distance from the outer edge. Length, 8 mm. width, 13 mm. The species is named Eurypterus thomasi in honor of Prof. A. O. Thomas
