A collection of rhabdocoels from Douglas Lake, Michigan, kindly sent to me for identification by Mr. George Moore, of the zoology department of the University of Michigan, consisted, in addition to Gyratrix hermaphroditus hermaphroditis, stated by Mr. Moore to be the most common rhabdocoel in Douglas Lake (apart from Stenostomum, one supposes), of species of Macrostomum, Microstomum, Dallyellia, and Mesostoma. While probably most of these forms were new species, the material sufficed only for a determination of the Mesostoma, which is herein described.