Plants with basal sheaths of previous year persistent as linear fibers. Culms to 60 cm × 2 mm, usually scabrous abaxially. Leaves: proximal sheaths usually all with blades, fronts hyaline, smooth, apex colorless, hyaline, concave, entire; ligules obtuse, 5 mm, free limb to 0.2 mm; blades clustered at base, not epistomic, to 60 cm × 4 mm. Inflorescences very condensed, ovoid to shortly cylindric, with 5-10 individually indistinguishable branches, to 2.5 × 1.5 cm; proximal internodes not visible, not more than 3 mm; proximal bract scalelike, inconspicuous. Scales hyaline, dark brown, subequal to perigynia, acute. Perigynia pale brown, 7-11-veined abaxially, 5-7-veined adaxially, to 3.5 × 1.5 mm, base not distended proximally, rounded or cordate; stipe to 0.1 mm; beak to 1.5 mm, smooth or subserrulate, apex entire, oblique or, sometimes, bidentate with teeth to 0.1 mm. Achenes ovate, to 1.5 × 1 mm; persistent style base cylindric.
The affinities and sectional placement of Carex jonesii are unclear. Although C. jonesii has often been considered to be part of the C. nervina-C. neurophora complex, it is distinguished from those species by numerous vegetative and reproductive characteristics, including basal leaves with short sheaths with rapidly disinte-grating hyaline fronts and perigynia with smooth beaks, oblique, rather than bidentate at the mouth. Carex jonesii is frequently confused with other western montane sedges that have capitate infloresences. It is most often confused with C. illota due to the strong similarity of the perigynia (somewhat shorter and more rounded apically in C. illota). Although C. illota is placed in sect. Ovales based on the gynecandrous spikes, that character can be very difficult to determine in mature plants due to the condensed inflorescence. The ovate, spongy-based perigynia of C. illota suggest a closer relationship with C. jonesii than with typical members of sect. Ovales.