Species description:
Genus name referring to the feather-like growth form. Species name meaning military plume, in reference to its resemblance to the feather on a soldier's hat.

Comments:
Sometimes called Hypnum crista-castrenesis, a synonym. Commonly called the knight's plume moss.

Distinguishing characteristics:
The regular, feather-like plants with the branches almost at right angles to the main shoot, the pale yellow­green colour tending to golden in sunny sites, and the pleated falcate­secund leaves that point toward the base of the main shoot, are distinctive.

Habit:
Forming mats of pale, yellow-green to golden green, regularly branched shoots that may be suberect or reclining.

Similar Species:
Within the genus Hypnum there are several species superficially similar to Ptilium. H. subimponens and H. callichroum are often regularly branched and yellow-green but the falcate-secund leaves are not pleated and point to the underside of the stem rather than to the stem base as in Ptilium. H. procerrimum sometimes shows the same regu­lar branching but plants are not yellow-green but tend to reddish­ brown. The regular branching of Ctenidium schofieldii is also similar to Ptilium, but C. schofieldii is reddish-brown to dark green and the main stem leaves are conspicuously squarrose. In calcareous marshy habitats Cratoneuron commutatum is sometimes regularly branched but forms dense mats of erect, rather than creeping plants; Ptilium is always in well-drained sites, usually in· forests. Some specimens of Drepanocladus uncinatus may resemble Ptilium but the Drepanocladus has a single mid­rib and the branch leaves curve downward toward the soil surface rather than in the direction of the base of the main shoot, which is characteristic for Ptilium.