336 ON LYELLIA, LBPTOSTOMUM, 



however, in several specimens of Polytrichum 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 JDawsonia ; 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 

 666] 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- 

 tricho'idece, and is wanting in Polytrichum undulatum 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 Dawsonia ; 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, IcBvigatum} and since noticed by Messrs. Hooker and 

 Taylor ^ as existing in nearly the whole of the genus, do not 

 belong to the nerve only, as the authors of Muscologia 

 JBritannica 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 



' Flora Lappon. tab. 22. ' Muscol. Brit. p. 24. 



