Plants mostly coarse, forming loose tufts or mats, dark or yellow- to olive green or yellow-, olive, rusty to blackish brown distally, brown, reddish or blackish brown proximally, sometimes black or brown throughout. Stem (1-)3-11(-15) cm, repeatedly forked or sparsely to copiously dichotomously or fasciculately branched. Leaves erect-appressed to imbricate, often secund on drying, erect-spreading on wetting, lingulate, elliptical, broadly ovate, ovate- or oblong-lanceolate to broadly lanceolate, (1.5-)2.2-2.9(-3.2) × (0.8-)1-1.2 mm, margins recurved on both sides in the proximal half, plane, erect or somewhat inflexed in the distal part, 1-stratose, entire for about 3/4-4/5 of the distance to the apex, irregularly remotely, bluntly or sharply toothed, erose-dentate to sinuate at the apex; apices acute, subacute, rounded to broadly rounded-obtuse and muticous; costa subpercurrent, entire or shortly forked at tip, (60-)70-160(-180) µm wide near the base; laminal cells 1- or very rarely 2-stratose distally, papillose or exceptionally smooth. Inner perichaetial leaves hyaline to yellowish hyaline, sometimes with chlorophyllose, thick-walled cells in the distal half or with a small group of chlorophyllose cells at the apex. Seta blackish or reddish brown, (1.8-)4-15(-17) mm. Capsule brown, ovoid, obloid to shortly cylindric, (1-)1.5-2.8(-3.1) mm, smooth; peristome teeth lanceolate, (300-)350-450(-500) µm, yellowish brown to dark reddish brown, densely low- or spiculate-papillose, 2-fid or tripartite down to the middle or two thirds of their length. Spores (10-)15-20 µm.
Moist or wet, sometimes periodically dry, shaded acidic or rarely calciferous rocks, boulders, cliffs, slabs and blocks in stream beds or close to brooks and rivers periodically washed by wave action, in seasonal creeks and on lake shores, often permanently submerged in the rapids of streams and waterfalls; low to high elevations (0-3300 m); B.C., N.B., Nfld. and Labr., N.S., Ont., Que., Yukon; Ala., Alaska, Ariz., Calif., Colo., Conn., Ga., Idaho, Ky., Maine, Md., Mass., Mich., Minn., Mont., Nev., N.H., N.J., N.Y., N.C., Pa., R.I., S.C., Tenn., Vt.,Va., Wash., W.Va., Wis.; Europe; temperate Asia; n Africa; Atlantic Islands (Azores, Canary Islands, Iceland, Madeira).
Codriophorus acicularis is a boreal-montane species, with a marked oceanic affiliation. It is bicentrically distributed in North America and, apart from C. fascicularis, is the most common species of the genus on the continent. In western North America it extends along coastal areas from the Aleutian Islands to central California. It recurs in the Rocky Mountains, with highly isolated stations in northern Colorado, where it reaches its highest altitude of 3292 m, and in southern Arizona. In eastern North America it has a continuous range from south-eastern Labrador to northern Georgia and Alabama, with two isolated stations on the Ozark Plateau of Arkansas and Oklahoma. In this part of the continent it reaches its highest elevation of 1525 m in the Great Smoky Mountains of Tennessee.
Codriophorus acicularis is central to a group of four species constituting the typical subsection of the genus; all occur in the flora area (H. Bednarek-Ochyra 2006). It is characterized by its percurrent costa that is unbranched or spurred and is situated at the bottom of a shallow, wide-angled, and open groove. In transverse section it is 2-3-stratose in the distal and medial parts, 3-6(-7)-layered in the basal part, and flattened to lunate and distinctly convex on the abaxial side. Moreover, the leaf margin is irregularly bluntly or sharply and coarsely dentate, erose-dentate, or only sinuate in the upper 1/5-1/4.