Plants in flat patches, blackish green. Stems 1--1.5 cm. Stem leaves homomallous-falcate, oblong-lanceolate, from a clasping base tapering to a long, subulate uncinate point, 2--3.5 mm, margins plane, erect above, intermarginal bands absent, usually muticous, occasionally short hyaline awns are present, not decurrent, costa narrow proximally; distal laminal cells 2-stratose, irregularly rounded; medial laminal cells irregular rounded, thick-walled; basal juxtacostal laminal cells short-rectangular, straight, thick-walled; basal marginal laminal cells quadrate, thick-walled. Perichaetial leaves not enlarged. Seta straight, 2.5--3.5 mm. Capsule occasionally present, exserted, obloid, shiny, smooth becoming striate when dry, stomates absent, annulus absent, operculum conical, peristome split in distal half, basal segments smooth, distal segments papillose. Calyptra unknown.

Dry rock and boulders; of conservation concern; 1500--3500 m; Calif., Oreg., Wash..

Grimmia hamulosa is a rare endemic to the Pacific Coast states of western North America. It has been poorly collected, its habitat is not well defined, and it is a somewhat confusing species. The original description is incomplete and partly incorrect, e.g. it lacks discussion and comparison with related taxa, and the seta is straight, not bent, as stated in the protologue. Grimmia hamulosa is characterized by (1) a glossy habit with long tapering homomallous muticous leaves with plane margins, (2) a broad, weakly outlined costa that fills the distal part of the 2-stratose lamina, and (3) glossy exserted capsules without an annulus on a straight seta. Despite its unique leaf characters and lack of an annulus Hastings puts it in subg. Litoneuron based on its thick, concave leaves with plane margins, costa not projecting prominently from the lamina, and its exserted capsule on a straight seta. In 1999, H. C. Greven found G. hamulosa richly fruiting in Yosemite National Park. Although isotypes studied by Greven have muticous leaves, in some of the plants from Yosemite the leaves have inconspicuous short awns. However, these plants do not deviate in other aspects from the type specimen. Small specimens of this species resemble G. fragilis Schimper, a form of G. montana described from southern Europe. These plants are characterized by long, glossy leaves usually with broken tips. In G. hamulosa, the leaf tips are also frequently broken, however, in G. montana the costa is small and clearly defined, and the distal leaf margins are incurved.