Protologue Description: JUNCUS pervetus, n. sp., radicibus, rhizoma, culmis et foliis ut apud J. Roemerianum; culmis teretibus vel compressis rigidis rectis vel tortis 0.6-1 m. altis; inflorescentiis terminalibus vel pseudolateralibus supra compositis 2.5-10 cm. longis 2.5-6.5 cm. diametro, ramis rigide adscentibus vel divergentibus; capitulis 10-30-floris subsphaericis segregatis; floribus circa 2 mm. longis interdum unisexualibus; sepalis petalisque subaequalibus pallide fuscis vel albiscentibus oblongis obtusis membranaceo-marginatis, sepalis subcarinatis; staminis 6 interdum abortivis, filimento antheram, aequante; fructo exserto 3 mm. longo ovato-prismatico subulato-attenuato nitido stramineo vel rufescente, placentis non crassatis; seminibus 0.4-0.6 longis anguste obovoideis apice obtuse mucronatis basi breviter albido-caudatis.
Roots, rhizome, culms and leaves as in J. Roemerianus: culms terete or compressed, rigid, erect or twisted, 0.6-1 m. high: inflorescences terminal or falsely lateral, much branching, 2.5-10 cm. long, 2.5-6.5 cm. in diameter; branches rigid, acsending or divergent: heads 1-30-flowered, subspherical, scattered: flowers about 2 mm. long, sometimes unisexual: sepals and petals subequal pale fuscous or whitish, oblong, obtuse with membranaceous margins; sepals somewhat acrinate: stamens 6, swometimes wanting: filimets equalling the anthers: fruit exserted, 3 mm. long, ovate-prismatic, subulate-attenuate, shining, stramineous or rufescent: the placentae not thickened: seeds 0.4-0.6 mm. long, narrowly obovoid, bluntly mucronate at apex, with short white caudate appendage at base. - MASSACHUSETTS: brackish swale, Hyannis, August 29, 1909, E.W. Sinnott (distributed as J. articulatus, var. obtusatus); forming a dense swale for a few rods only at the peaty upper border of a brackish marsh, east side of Lewis Bay, Yarmouth, October 14, 1916, M.L. Fernald & F.K. Butters, no. 15,064 (TYPE), also in Plantae Exsiccatae Grayanae.