1	Leaves ending in ± serrate, hyaline awn.		Orthotrichum diaphanum
+	Leaves obtuse, acute, acuminate, or cuspidate, not ending in hyaline awn.		(2)
2 (1)	Leaf margins erect, incurved, or involute; dioicous; leaves ovate or elliptic, with broadly obtuse or rounded apices; calyptrae papillose, not plicate or hairy.		(3)
+	Leaf margins plane, recurved, revolute, or thickened; condition autoicous or if dioicous then leaves acuminate; leaves ± lanceolate or oblong, with narrowly obtuse, acute, or acuminate apices; calyptra smooth or rarely papillose, plicate, hairy, or naked.		(4)
3 (2)	Leaf margins involute; distal leaf cells with 2--3 conical papillae per cell; peristome lacking; Newfoundland.		Orthotrichum gymnostomum
+	Leaf margins erect-incurved to incurved; distal leaf cells with 1 papilla per cell; peristome double; common throughout boreal and western regions.		Orthotrichum obtusifolium
4 (2)	Leaves crisped-flexuose when dry; capsule cylindric or elliptic-cylindric, fully exserted; stomates immersed; Pacific Northwest and Alaska.		(5)
+	Leaves erect-appressed when dry; capsules various; stomates superficial or immersed; throughout North America.		(6)
5 (4)	Plants 0.8--2 mm; distal leaf cells 7--11 µm wide; seta usually 2.5--6 mm; capsule long-exserted, ovate-cylindric when mature; peristome teeth 8, light-brown, finely papillose on the exterior, finely striate on the interior; stomates immersed, in the middle and proximal portion of the capsule.		Orthotrichum consimile
+	Plants usually less than 1.3 cm; distal leaf cells 9--14 µm wide; seta 1.5--3.5 mm; capsule barely exserted, oblong-cylindric when mature; peristome teeth 16, red, irregularly papillose-reticulate; stomates immersed above the middle of the capsule, never in the proximal portion.		Orthotrichum pulchellum
6 (4)	Stomates superficial; basal leaf cells elongate, ± nodose, thick-walled.		(7)
+	Stomates immersed; basal leaf cells rectangular, not nodose, thin-walled.		(26)
7 (6)	Endostome segments well-developed, usually present when capsule is old; exostome teeth recurved or reflexed; prostome never present; leaves sometimes flexuose when dry, always 1-stratose; usually on trees.		(8)
+	Endostome segments rudimentary or, more often, absent; exostome teeth erect, sometimes reflexed when old and dry; prostome usually present; leaves stiff, erect-appressed when dry, rarely 2-stratose; almost always on rock.		(16)
8 (7)	Exostome teeth reflexed; endostome segments 8, narrow, consisting of a single row of cells; capsule strongly 8-ribbed at least 1/2 length of capsule.		(9)
+	Exostome teeth recurved; endostome segments 8 or 16, large, stout, and thick, consisting of 2 rows of cells; capsule smooth or weakly 8-ribbed no more than 1/2 length of capsule or if strongly 8-ribbed, then dioicous.		(10)
9 (8)	Exostome fenestrate and cancellate; endemic to St. Paul's Island, Alaska.		Orthotrichum fenestratum
+	Exostome not fenestrate or cancellate, at most perforate at tips; Arctic south to California.		Orthotrichum pylaisii
10 (8)	Calyptra oblong-conic, naked; endostome segments hyaline, delicate, as wide as exostome teeth; capsule widest at mouth, ovate; leaves oblong-ovate, rounded-obtuse; plants never longer than 1 cm, largest at apex.		Orthotrichum exiguum
+	Calyptra short-conic, hairy; endostome segments yellowish, stout, not as wide as exostome teeth; capsule widest at middle or cylindric; leaves lanceolate, acute or acuminate; stems longer than 1 cm, of uniform thickness.		(11)
11 (10)	Plants dioicous; exostome teeth acuminate; capsule long-cylindric and 8-ribbed.		Orthotrichum lyellii
+	Plants autoicous; exostome teeth acute or truncate; capsule of various types.		(12)
12 (11)	Leaves slenderly acute, tip contracted to large, cuspidate point; peristome teeth truncate, often united at base to form basal ring, endemic to Appalachian Mts.		Orthotrichum keeverae
+	Leaves acute or acuminate; peristome teeth acute, separate to base; not found in Appalachian Mts.		(13)
13 (12)	Exostome teeth 16, endostome segments 16, usually not erose; New Mexico and Arizona.		Orthotrichum pycnophyllum
+	Exostome teeth 8, or if 16 endostome erose; endostome segments 8 to 16; temperate and boreal regions southward to California and Missouri.		(14)
14 (13)	Capsule lightly 8-ribbed.		Orthotrichum speciosum
+	Capsule smooth.		(15)
15 (14)	Capsule cylindric, exserted; exostome teeth 8; endostome segments 8.		Orthotrichum elegans
+	Capsule ovate, immersed; exostome teeth 16; endostome segments 16.		Orthotrichum striatum
16 (7)	Leaves 2-stratose, margins plane; plants dark-green to black.		Orthotrichum bolanderi
+	Leaves 1-stratose, margins revolute; plants light- to olive-green.		(17)
17 (16)	Peristome rudimentary, appearing absent.		Orthotrichum spjutii
+	Peristome present.		(18)
18 (17)	Exostome teeth smooth; leaves smooth; calyptra naked; prostome large, extending 1/3 way up exostome; seasonally wet rocks.		Orthotrichum holzingeri
+	Exostome teeth papillose; leaves papillose; calyptra hairy; prostome small or absent; dry rocks.		(19)
19 (18)	Capsule cylindric, fully exserted.		(20)
+	Capsule usually oblong and emergent, if cylindric and exserted then strongly 8-ribbed.		(21)
20 (19)	Exostome teeth usually 8, reflexed-recurved; capsule lightly 8-ribbed, exothecial cells differentiated for 1/2 length of capsule; endostome usually present in at least some capsules.		Orthotrichum pylaisii
+	Exostome teeth usually 16, erect; capsule smooth or slightly plicate; exothecial cells not or scarcely differentiated; endostome rudimentary, usually absent in open capsules.		Orthotrichum laevigatum
21 (19)	Exostome teeth erect; endostome absent.		Orthotrichum rupestre
+	Exostome teeth reflexed; endostome present.		(22)
22 (21)	Capsule ovate to ovate-oblong, fully exserted; exostome teeth 16.		(23)
+	Capsule oblong to cylindric, emergent to slightly exserted; exostome teeth 8.		(24)
23 (22)	Exostome fenestrate and cancellate; endemic to St. Paul's Island, Alaska.		Orthotrichum fenestratum
+	Exostome not fenestrate or cancellate, at most perforate at tips; Arctic south to California.		Orthotrichum pylaisii
24 (22)	Capsule oblong, ± 1/2 emergent; exostome teeth perforate only at tips, not cancellate; endostome segments usually not present when capsules are old and dry; leaves gradually narrowed to a long acuminate-cuspidate apex, which is ± colorless; rare, western states, saxicolous.		Orthotrichum praemorsum
+	Capsule long-cylindric to oblong, 1/2 emergent to exserted; exostome teeth deeply perforate and cancellate, each tooth appearing as split into 4 vertical rows; endostome segments usually present when capsules are old and dry; leaves acute; common, throughout northern North America, corticolous.		(25)
25 (24)	Capsule long-cylindric to cylindric, greater than 1.5 mm, ribbed entire length, emergent over 1/2; tapering gradually to seta, which is usually over 1 mm in length; endostome papillose; western North America.		Orthotrichum affine
+	Capsule oblong to oblong-cylindric, less than 1.5 mm, ribbed for ± 1/2 length of capsule, ± emergent; tapering to seta through a long, shriveled neck, which is less than 1.5 mm in length; endostome smooth; Great Lakes area, northeastern North America, and Alaska.		Orthotrichum sordidum
26 (6)	Leaves ± obtuse; margins coarsely dentate near apex; calyptra naked, distinctly plicate, papillose, peristome of 8, reflexed teeth; rocks or bases of trees near streams, sometimes submerged; west of continental divide.		Orthotrichum rivulare
+	Leaves ± acute or narrowly obtuse; margins entire or denticulate near apex; calyptra naked or hairy, not distinctly plicate, smooth; peristome various; trunks of trees or dry rock faces; variously distributed.		(27)
27 (26)	Exostome teeth erect or spreading, striate, reticulate, or papillose-striate; capsule with 8 or 16 ribs.		(28)
+	Exostome teeth reflexed or recurved, papillose, rarely striate at tips; capsule smooth or 8-ribbed.		(33)
28 (27)	Endostome segments 16, linear, papillose, as long as exostome.		Orthotrichum underwoodii
+	Endostome segments absent, or 8 short, not well-developed.		(29)
29 (28)	Capsule fully exserted, cylindrical, usually with 8 long and 8 short ribs; leaves 1-stratose.		Orthotrichum anomalum
+	Capsule immersed or emergent, rarely shortly exserted, ovate or oblong, usually with 8 or 16 ± uniform ribs; leaves 1- or 2-stratose.		(30)
30 (29)	Capsule with 16 ribs, immersed to 1/3 emergent, ovate and constricted below mouth when dry; leaves ovate-lanceolate, acute, 1-stratose or rarely with 2-stratose streaks.		Orthotrichum cupulatum
+	Capsule with 8 ribs, ± emergent, oblong or oblong-ovate, sometimes constricted below mouth when dry; leaves lanceolate to ligulate, obtuse or blunt, 1- or 2-stratose.		(31)
31 (30)	Exostome teeth finely papillose or finely reticulate-papillose, 16, yellowish, erect or spreading-recurved; leaf margins 1--5 cells thick, at least in some leaves thicker than lamina; capsule usually strongly constricted below mouth when old; eastern North America.		Orthotrichum strangulatum
+	Exostome teeth ridged-striate, coarsely striate-reticulate, or coarsely papillose-striate, 8 splitting to 16, white to clear, incurved, spreading, or reflexed; leaf margins not thicker than lamina; capsule only moderately constricted below mouth when dry; western North America.		(32)
32 (31)	Leaves 2-stratose or partially 2-stratose, green, lanceolate, obtuse, loosely-appressed, not incurved when dry; papillae small and conical; exostome teeth usually 8.		Orthotrichum hallii
+	Leaves 1-stratose, usually thick and glaucous, ligulate, blunt, stiff, incurved when dry; papillae large, 2-3 forked; exostome teeth ± 16.		Orthotrichum pellucidum
33 (27)	Capsule smooth when dry; exostome of 16, erect to recurved teeth; endostome absent.		Orthotrichum pusillum
+	Capsule 8-ribbed when dry; exostome teeth 8, or 8 splitting to 16, reflexed; endostome usually present.		(34)
34 (33)	Leaf apex cuspidate and usually denticulate, margins incurved and forming channel at apex; southwestern U.S.A.		Orthotrichum bartramii
+	Leaf apex obtuse, acute, or apiculate, margins not forming channel at apex; throughout North America.		(35)
35 (34)	Stomates scarcely to 1/2 covered by non-projecting subsidiary cells, inner walls not much thickened; calyptra usually naked, oblong to conic-oblong.		(36)
+	Stomates 1/2 to completely covered by projecting subsidiary cells, inner walls thickened; calyptra usually with some hairs, oblong.		(38)
36 (35)	Leaves acute or narrowly obtuse, never apiculate, oblong-lanceolate to lanceolate; distal leaf cells less than 14 µm; calyptra conic-oblong.		Orthotrichum pallens
+	. Leaves apiculate or rounded-obtuse, ovate or narrowly ovate-lanceolate; distal leaf cells greater than 13 µm; calyptra oblong.		(37)
37 (36)	Leaves ovate to ovate-oblong, rounded-obtuse, at least some leaves mucronate, carinate; western North America.		Orthotrichum flowersii
+	Leaves ovate-lanceolate to lanceolate, acute, most leaves apiculate, not carinate; throughout North America.		Orthotrichum pumilum
38 (35)	Leaves with 2- to 3-stratose margins; dry granite rocks.		Orthotrichum shevockii
+	Leaves with 1-stratose margins; trees and mesic rocks.		(39)
39 (38)	Exostome teeth with striate-reticulate ornamentation in distal portion; calyptra with strongly papillose hairs; leaf cells papillose, with regular, long-forked papillae; leaves slightly twisted when dry; capsule strangulate when dry.		Orthotrichum alpestre
+	Exostome teeth papillose; calyptra hairs smooth or papillae formed by projecting cell walls; leaf cells smooth or with low, conical papillae, papillae never forked; leaves stiff, erect-appressed when dry; capsule variously constricted beneath mouth when dry.		(40)
40 (39)	Vaginula with numerous hairs.		Orthotrichum stramineum
+	Vaginula without hairs.		(41)
41 (40)	Leaf tips incurved when dry; capsule cylindric to cylindric-elliptic, not constricted beneath mouth, but contracted along entire length; stomates in proximal 1/3 of capsule, completely covered by subsidiary cells; restricted to west coast.		Orthotrichum tenellum
+	Leaf tips not incurved when dry; capsule oblong-cylindric, constricted beneath mouth, never contracted along entire length; stomates at middle of capsule; Rocky Mountains and eastward.		(42)
42 (41)	Leaf cells greater than 14 µm, the lumina smooth and rounded; leaves apiculate or slenderly acute, ovate-lanceolate to oblong-lanceolate; subsidiary cells not completely covering stomates.		Orthotrichum pumilum
+	Leaf cells less than 14 µm, lumina irregular due to uneven wall thickenings; leaves broadly acute to obtuse, lanceolate; subsidiary cells completely covering stomates.		(43)
43 (42)	Capsule strongly constricted beneath mouth, brown; exothecial cells with thickened walls, in rows 6--8 cells wide; exostome teeth 8; subsidiary cells projecting horizontally over stomate.		Orthotrichum stellatum
+	Capsule not constricted beneath mouth, yellow; exothecial cells with thickened walls in rows 4--6 cells wide; peristome teeth 16, rarely united into 8 pairs; subsidiary cells projecting upward over stomate.		Orthotrichum ohioense