Scouleria W. J. Hooker, 1829 [1830].

Scouleria is one of a large number of mosses growing in rapidly flowing, unpolluted rivers and streams. Perhaps as a survival strategy these seasonally submerged mosses have evolved a differentiated, thick-walled margin that seems to resist tearing in the current. Another probable adaptation is that the capsules are almost sessile and the capsule wall is of many cell layers. Scouleria is especially obvious when dry because it forms jet-black mats on rocks exposed during late summer.

Key to Scouleria Etc.

The mosses treated in this section are aquatic or semiaquatic plants with large and broad leaves, obtuse or rounded at the apex. The marginal cells of the leaves are quadrate like the median laminal cells, but those marginal cells are larger and thick-walled forming an opaque border easily seen under the dissection microscope.

Species included in this key are in Scouleriaceae, except Crumia and Scopelophila (Pottiaceae), and Schistidium (Grimmiaceae):
Crumia latifolia (Kindberg ex Macoun) W. B. Schofield
Schistidium rivulare (Bridel) Podpera
Scopelophila cataractae (Mitten) Brotherus
Scouleria aquatica W. J. Hooker in Drummond
Scouleria marginata E. Britton

A. Margin entire to apex; limbidium present as a one cell wide, unistratose border of enlarged and thicker-walled cells .....B
A. Margin serrate to dentate; margin mostly of several layers of cells, at least near leaf apex .....C

B. Median laminal cells smooth, quadrate, to 12 µm in diameter; leaves mostly less than 3 mm long .....Scopelophila: S. cataractae
B. Median laminal cells papillose, hexagonal, > 20 µm in diameter; leaves mostly more than 5 mm long .....Crumia: C. latifolia

C. Rhizoids never inserted on abaxial surface of costa; alar region with a cluster of isodiametric, rather thin walled cells extending toward the costa in 8 or more rows .....Schistidium: S. rivulare
C. Rhizoids abundantly inserted on adaxial surface of costa; alar region usually with 1-3 rows of quadrate cells set off by an infra-marginal limbidium of elongate and rectangular cells that border the shorter cells inward toward the costa .....D

D. Capsules gymnostomous; leaf margin 3-5 cells thick above the leaf middle with internal cells of that thickened margin substereid .....Scouleria marginata
D. Capsules peristomate; leaf margin at most 2 cells thick without substereid cells internally .....E

E. Leaves bi- to multistratose in most of leaf; leaf shape somewhat ovate-lanceolate ..... Scouleria species A
E. Leaves uniformly unistratose or with scattered bistratose streaks in the central lamina and on the margin; leaf shape elliptic, broadest near the middle .....Scouleria aquatica