This species has long been known as Didymodon trifarius (Hedw.) Roehl. The type of that species, however, actually represents Saelania glaucescens (Hedw.) Broth. (see Zander 1978a). Didymodon luridus is a small species with short-lanceolate, ovate-lanceolate, or triangular-lanceolate leaves that have obtuse or broadly acute apices, smooth to weakly papillose leaf cells, and a broad costa that ends well below the apex. There is a Guatemalan collection (Standley 65640, F) that fits uncomfortably in D. luridus because its leaf cells are somewhat large, extremely irregular in shape and at times unipapillose.

In most of its features D. luridus closely resembles D. tophaceus, and most Central American reports of D. tophaceus are instead D. luridus. Didymodon luridus differs from D. tophaceus in having quadrate instead of elongate cells on the upper ventral surface of the costa and not to slightly decurrent leaves. The last feature, however, is not always reliable in separating the two species since according to Zander (1998) long leaf decurrencies are absent from a large percentage of D. tophaceus collections. Didymodon nigrescens is similar to D. luridus in having obtuse to broadly rounded leaves, short costae, and quadrate cells on the upper ventral surface of the costa. It differs from D. luridus in its consistently reddish yellow color, lack of a stem central strand, leaves with long, bulging decurrencies, and costae that are more or less homogeneous in cross-section.

The world distribution of D. luridus is difficult to determine because the species has been  confused with other Didymodon species. For example, Zander (1981), citing specimens from Washington, Arizona, and California, attributed western North America to its range. But, he later (1993, 1998) excluded it from North America north of Mexico and considered all reports of D. luridus from the area to be D. brachyphyllus (Sull.) Zand., D. nicholsonii Culm, D. tophaceus, or D. vinealis.