336 ON LYELLIA, LEPTOSTOMOI,
however, in several specimens of PolytHchum alpinum still more minute pustules, not very different in appearance, and similarly situated on the base of the capsule.
In establishing this new genus of Mosses, it is of import- ance to determine its more intimate affinities in the family to which it belongs. Its place is unquestionably between Polytrichum and Dawsonia ; and it will I believe be admit- ted, that these three genera, in the natural method, cannot be separated ; though they will necessarily form or be re- ferable to distinct sections of an artificial system founded chiefly on modifications of the peristomium.
In attempting to discover characters by which this group of PolytrichoidetE may be distinguished from other Mosses, it is in the first place necessary to determine the whole structure of Polytrichum ; for this genus, though one 566] of the most common of the order, and, from the great size of the capsules in many of its species, most readily admitting of accurate observation, has never yet been thoroughly examined.
One of the most striking characters of Polytrichum is the dense texture and consequent opacity of the leaves ; in which it agrees with the other two genera of the section. This character, however, is not altogether confined to Poly- trichoidece, and is wanting in Polytriclium undidatum and angustatum. But the lamellae of the upper surface of the leaves probably exist, though in very different degrees, in all the species of Polytrichum ; are equally observable in Lyellia and Daivsonia ; and I am not aware that they have been found in any other genera of the order.
These lamellae, which are represented in several of the species figured in English Botany, by Wahlenberg in P, lavigatum} and since noticed by Messrs. Hooker and Taylor 2 as existing in nearly the whole of the genus, do not belong to the nerve only, as the authors of Muscologia Britannica seem to suppose, but in several species cover the greater part of the surface of the upper or spreading por- tion of the leaf ; the sheathing base being either entirely
1 Rora Lappon. tab. 22. 2 Muscol. Brit. p. U.
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