A new species of leiobunine harvestman from the Chiricahua Mountains of Arizona is described. The species lacks pro- and retrolateral submarginal rows of coxal denticles, a feature often considered diagnostic for the polyphyletic Nelima, and has greatly reduced ventral dentition on the palpal claw, as in the monotypic Leuronychus. In most other respects, the species is uniquely similar to members of a clade from central and western Mexico currently in the poly- and/or paraphyletic Leiobunum. These traits include a supracheliceral lamina with a wide transverse plate and a canaliculate ocularium, with an anterior surface that slopes dorsoposteriorly and a posterior surface that bulges rearward and is constricted at its base. There is thus a conflict between classification using traditional diagnostic characters and classification using unique similarity of non-traditional characters. The problem is exacerbated by the problematic status of each candidate genus. Here the species is placed in Leiobunum as L. silum sp. nov., a decision that gives weight to probable phylogenetic affinity with species currently placed in that genus. Leiobunum silum provides an excellent example of the limits of traditional typological classification and the need for a broad-scale morphological and molecular revision of sclerosomatid harvestmen.