Forty-five species of lower Middle Ordovician trilobites, including six new genera and thirty new species, are described and illustrated from the extensive collections in the United States National Museum. Most of these species occur in beds whose age and correlation have been a matter of controversy for more than a decade. The stratigraphic evidence afforded by the trilobites lends strong support to regional stratigraphic interpretations of the lower Champlainian beds in the Appalachian Valley, which have been worked out by G. Arthur Cooper and the writer fromdetailed study of brachiopod faunas and from physical stratigraphic studies. The trilobites, like brachiopods, are not so restricted in their facies distribution as are many groups of invertebrate fossils. Hence they are very useful in establishing contemporaneity of dissimilar facies. In this paper only the more common trilobites are described. The principal purpose of this study is to make available for biostratigraphic use a number of trilobite species, most of which have been confused or misidentified previously.