This northern "rock elm" has a somewhat fragmented range from New England to the midwest, and it occurs only on calcareous soils. There has been confusion in Ky., Tenn. and elsewhere with serotina or other species. Most previously published records from Tenn. (Little 1971, FNA 3, Ch) are erroneous; colls. from Scott and White Cos. (TENN) may be correctly identified. In Ky. further verification is needed for several counties, especially BELL (Gm; Little 1971). There are relatively few flowering or fruiting colls. of thomasii (Mar-May) and serotina (Aug-Nov), in contrast to the other native elms (Feb-Apr). Compared to americana, leaf blades of both thomasii and serotina tend to have more planar arrangement on branches (versus declined); they are often broader in shape (L/W mostly 1.7-2.2 versus 2-2.5) and sometimes obovate (versus ovate), with a more short-attenuate to acuminate apex (versus gradually tapering from near the middle), uniformly soft-hairy on lower surfaces (versus hairy to glabrate but with distinctive tufts in axils), and usually (at least in mature trees of Ky.) more or less glabrous above (versus often scabrous). In Ky. at least, leaves of thomasii tend to have broader shape than americana, closer secondary veins in sun (mostly 3-5 mm apart versus 5-7 mm), and more distinct tertiary veins (versus less distinct from ultimate areolae). Further west (from Mo. to Iowa), leaves of thomasii on vegetative shoots tend to have less closely spaced secondary veins and tend to be more scabrid above. Although irregular corky wings occur on branches of thomasii and serotina, americana can also become somewhat corky, especially on drier sites. U. thomasii often appears to send up suckers from lateral roots. Also, it does not appear to suffer significantly from Dutch Elm Disease in Ky.