Stems commonly unbranched. Leaf base distinctly and rather abruptly wider than the limb. Sexual condition dioicous. Seta 0.8--1.2 cm. Capsule with theca usually 2--2.8 mm, operculum 0.6--1 mm, peristome not or slightly twisted.

Sporophytes mature spring--summer. Soil, clay, roadside banks; 150--1500 m; B.C.; Ariz., Calif., Mont., Oreg., Wash.; Pacific Islands (Hawaii); restricted to western North America.

This species is very similar to the European Timmiella flexiseta (Bruch) Limpricht, and commonly matches the key traits (stems to ca. 1 cm, leaf margins dentate only near apex, peristome teeth short-filiform, erect) of T. diminuta (J. K. A. Müller) P.-C. Chen of Asia as given by Li Xing-jiang et al. (2001). Revision may result in synonymy. Sterile plants of the two species of the flora are not easily distinguishable even by the leaf base characteristics (see discussions by R. H. Zander 1994 and D. H. Norris and J. R. Shevock 2004). Although several authors indicate that T. crassinervis differs from T. anomala by a peristome that is straight while that of the latter is twisted at least once, E. Lawton (1971) described the peristome as "slightly twisted" and distinguished T. crassinervis by its smaller size, unbranched stem, and dioicous condition. According to D. H. Norris and J. R. Shevock (2004), in California where the two species co-occur, T. crassinervis is more common in the humid northwest of the state.