Collections made in certain springs and seepage areas in Michigan, Illinois, and Minnesota during the summers of 1950-52 have turned up specimens of an interesting new genus of Thyasinae. The genus is named in honor of Ruth Marshall for her outstanding contributions to the study of North American hydrachnids. The author wishes to thank Mr. Rodger Mitchell of the University of Michigan, Ann Arbor, for the opportunity to examine some of his specimens belonging to this genus.