A. Leaf limb between the lamellae and the margin 1 cell
thick except the elongate border cells when present; capsule with
peristome.
B. Lamellae on upper side of leaf 1-15, sometimes none on
most of the leaves of Atrichum crispum; about the middle of
the limb the unistratose region more than 8 cells wide; capsule not
longitudinally ridged, without hypophysis; calyptra smooth or nearly so; marginal cells of
the lamellae like the others, not thicker walled.
C. Leaves bordered with long narrow cells; capsules
without stomata............1. Atrichum.
CC. Leaves not bordered with long narrow cells; capsules
with stomata.........2. Oligotrichum.
BB. Lamellae 25-70 (except in Pogonatum pensilvanicum only
10-15); about the middle of the limb the unistratose portion 1-8 cells wide; capsule
longitudinally ridged in most species, with distinct hypophysis; calyptra
long-hairy, densely so except in Polytrichadelphys lyallii.
D. Capsule terete; calyptra densely long-hairy; capsule
with or without stomata; teeth of peristome 32 or these more or less completely
doubled.............................................................5. Pognatum.
DD. Capsule 4-6 angled; capsule with stomata; teeth of
peristome 64.
E. Capsule longitudinally 4-ridged, the two upper ridges
closer together; calyptra sparingly hairy and thus its body quite easily
seen.............................................................................4.
Polytrichadelphus.
EE. Capsule longitudinally 4-6 ridged, the ridges equally
spaced; calyptra densely long-hairy and thus its body
hidden.......................................................................................................6.
Polytrichum.
AA. Leaf limb between lamellae and margin 2 cells thick
throughout most of its area; capsule without peristome; calyptra smooth; marginal cells of
the lamellae like the others at least in thickening; plants known only from Greenland,
Alaska and British Columbia................................3. Lyellia.
