Thallus: foliose, 1-3(-5) cm in diam., adnate, subdichotomously to irregularly lobate lobes: irregular, somewhat elongate, +plane to slightly roughened (but not wrinkled), separate, 3-6 mm wide, 50-100(-120) µm thick; apices: rotund, entire, dentate to irregularly cut and lobulate, occasionally upturned upper surface: light to medium gray, usually dull, smooth to roughened but not wrinkled isidia: usually dense, laminal to marginal, usually squamiform (flattened), concolorous with the thallus internal anatomy: with upper and lower cortices consisting of a single layer of irregularly isodiametrical cells 5-9 µm in diam., internally with loosely interwoven chains of Nostoc and hyphae lower surface: light to medium gray, smooth, with scattered tufts of white hairs Apothecia: rare, laminal, sessile to substipitate, 0.5-1 mm wide disc: brown to red-brown, concave margin: thalline, concolorous with the thallus or tan, entire or isidiate exciple: euparaplectenchymatous, 35-70 µm thick hymenium: hyaline below and thinly brown above, 100-115 µm tall; paraphyses: unbranched, c. 1.5 µm wide, slightly inflated apically; subhymenium: pale yellow, 20-35 µm thick asci: cylindrico-clavate, 8-spored ascospores: hyaline, 3-5-septate transversely, 1-septate longitudinally, ellipsoid to subfusiform, 20-25 x 7-9 µm Pycnidia: not observed Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: particularly common on acidic rock at intermediate elevations World distribution: new world tropics into southwestern North America Sonoran distribution: central and southeastern Arizona, southern California, Baja California Sur, Sonora, Chihuahua, and Sinaloa. Notes: The description above corresponds to Sierk (1964) and is one of the most common Leptogiums on acidic rocks at mid-elevations in the Sonoran Region. Unfortunately this description does not correspond well to the type material (Jørgensen 1977), and an alternative name is not readily available. The marginal squamiform isidia (or phyllidia) are typical of this medium gray species, and separates it from L. cyanescens, which typically has cylindrical, laminal isidia. The occurrence of occasional intermediate forms has led some lichenologists (e.g. Degelius) to propose synonomy. However, the different morphologies separate fairly well geographically with L. cyanescens more commonly occurring in more humid warm temperate habitats and L. denticulatum occurring in more subtropical (Jørgensen 1977) and semi-arid habitats. Thus, keeping the species separated is advisable at this time.