Two new species of zyloiulid diplopods are described from the Pennsylvanian Upper Freeport coal at Linton, Ohio. One species, Xyloiulus bairdi, is apparently very similar to X. pstrossi (Fritsch) of Czechoslovakia. The other, Plagiascetus lateralis, differs sufficiently from all known xyloiulids to warrant inclusion in a new genus, characterized by the oblique segmental ornamentation which is confined to the lower sides. The new genus Nyranius is proposed for the Bohemian species N. costatus and N. tabulatus of Fritsch, xyloiulids in which the striation is not congruent across the segmental suture. Remarks on the nomenclatorial history, taxonomic characters, possible ordinal position, and geographic distribution of the family Xyloiulidae are given. The group is known so far only from about 24 species from the upper Pennsylvanian series of North America and Europe.