AND BUXBAUMIA. 337
destitute of them, or having them much less developed and strictly confined to the nerve.
In the form and position of the male flowers there is probably an absolute uniformity in the whole of this group : but the only peculiarities consist in the perigonial leaf or bractea being formed chiefly of the sheathing part of the stem-leaf ; and in the new shoot proceeding from the centre of the star-like cluster. In P. undulatum, however, the former character is by no means obvious, and the latter is not perhaps constant ; and both are only presumed to exist in Lyellia.
The double calyptra of Polytrichum, long considered as the essential character of the genus, equally exists in [567 Dawsonia. But this outer or spurious calyptra, formed of densely matted hairs originating from the vaginula and the apex of the inner cucullate calyptra, is wanting in several species of PolytricJium, in some of which, and in Lyellia, the true calyptra is furnished with a few hairs only, either scattered over the whole of its surface, or confined to its apex • while in others, as in PolytricJium undulatwn, it is nearly, and in P. mageUanicum and Icevigatum, entirely smooth.
Respecting the teeth of the peristomium of Polytric/ium , I have very little to add, except that in P. mageUanicum they appear to be eighty in number, which is a higher multiple of sixteen than has hitherto been noticed in this genus, and the greatest number that has yet been found in a single series in the order.
On the nature of the transverse membrane or tympanum of PolytricJium, I have formerly made some remarks in treating of Dawsonia, and have there considered it as the remains of the pulpy continuation of the columella, originally occupying the cavity of the operculum. But its uniform texture, as well as its exactly circular form and equal margin remain to be accounted for ; unless this regularity may be supposed to depend on the circular, and apparently cor- responding, aperture of the inner membrane.
Most authors have described the tympanum of Poly- trickum as minutely perforated. These pores I have never
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