Thallus: crustose, smooth, composed of small, rounded patches surface: greenish gray, slightly shiny cortex: cartilaginous corticiform layer of densely arranged parallel hyphae photobiont: cells: globose, 5-10 µm in diam. Apothecia: immersed-erumpent with zeorine margin, round, 0.2-0.5 mm in diam. disc: greenish gray to gray, margin: greenish to white hymenium: 50-60 µm; hypothecium: hyaline, 5-10 µm thick asci: broadly clavate, 40-50 x 18-28 µm, 1-spored ascospores: hyaline, richly muriform, ellipsoid, 30-40 x 15-20 µm Hyphophores: sessile on thallus, scale erect, 0.3-0.7 mm wide and 0.3-0.5 mm high, pale white to gray and often slightly translucent, often somewhat cucullate and usually with two acute, lateral projections conidia: cells of diahyphae sausage-shaped, 4-8 x 1.5-2.5 µm, intermingled algal cells 3-5 µm in diam. Spot tests: all negative Secondary metabolites: none detected with TLC; calcium oxalate crystals absent. Substrate and ecology: on leaves in rather exposed situations in tropical and subtropical wet and moist forests World distribution: neotropics Sonoran distribution: coastal Sinaloa. Notes: Of the many species now known in the genus Gyalectidium, only three have a smooth thallus and lack calcium oxalate crystals. Besides G. catenulatum, these are G. conchiferum and the corticolous G. yahriae Buck & Sérus. Gyalectidium conchiferum has marginal hyphophores which have a strongly curved line of insertion, while the hyphophores of G. yahriae are strongly modified to functionally resemble goniocystangia. Gyalectidium areolatum and G. filicinum have similar hyphophores as G. catenulatum, but differ in having areolate or verrucose thalli.