This widespread southeastern species is close to perplexum, and ca. 1-10% of colls. appeared somewhat intermediate to Isely (1983); see also Krings (2004). There has also been confusion in nomenclature with X humifusum; see notes under rotundifolium. D. glabellum has been considered to have distinctive hooked hairs on its stems, petioles and the veins of upper leaf surfaces; perplexum usually lacks such hairs or they are less dense, and its stems have mostly longer spreading hairs. Also, upper leaf surfaces of glabellum usually have a slightly silvery central band. It tends to occur in drier and less fertile soils. Thomas (2020) has, however, rejected differences in pubescence for identification. Instead he distinguished glabellum as follows (see also W): ventral margins of most loment segments straight to slightly convex (versus concave); leaves gradually reduced in size up the stem (versus abruptly reduced), with small leaves often extending well into inflorescences (versus rarely so); terminal leaflets lanceolate to broadly ovate (versus narrowly ovate to broadly elliptic-ovate), broadest nearer the base (versus nearer the middle). [But differences in leaflet shape may have been switched in the paper; see images of types.] It would be useful to review Ky. colls. using the treatment of Thomas, and to consider whether introgression exists (which Thomas did not mention).