DIPHYSCIUM. 51 



The quaint appearance of the capsule, surrounded with the large, scarious 

 perichEetial bracts, is totally different from that of any other of our mosses, and when 

 once seen cannot be again mistaken. It is however very different with barren plants, 

 which may easily be overlooked, or mistaken, even on closer examination, for a species 

 of Trichostomum, e.g.^ Tr. nitidum or Tr. tennirostre. This, indeed, has often 

 been done, and the var. acutifolium was recently described as a new species under the 

 name of Didymodon Camusi by Husnot (v. Mtisc. Gallica, pp. 80, 433). It may 

 however be recognised by the 2-3 layers of cells in the leaf, and by the broad, ill- 

 defined vanishing nerve, taken in conjunction with the broad, hardly acute apex of 

 the leaves, which are less strongly crisped and hardly shining when dry. 



The var. aaUifolittm graduates into the type by intermediate forms, but in its 

 extreme state, as I have found it freely in the Lake District, with tall, somewhat 

 branched stems and very long, narrow leaves, it is a well-marked form. Dr. Braith- 

 waite describes it as having the arista of the perichaetial bracts smooth, but I have 

 not found them noticeably different from those of the type. 



Schimper describes the outer, rudimentary teeth of the peristome of Diphyscium 

 foliosum as " very short, triangular, granulose, transversely jointed, not unfrequently 

 perforated in the middle, pale yellow, reddish purple at apex." Hardly any other 

 bryologist appears to have been able to detect them so far developed at least as to 

 render any such detailed description possible. 



GROUP B. ARTHRODONTE^E. 



Peristome teeth (when' present) thin, membranous, derived 

 from a single layer of cells of the sporogonium ; transversely- 

 barred. 



SUB-GROUP I. APLOLEPIDEM. 



Peristome teeth often forked above, at base composed of two 

 layers of plates ; in the outer layer a single plate forms the width 

 of the tooth ; in the inner two plates go to form the width of the 

 tooth, which therefore, when viewed from the interior, presents a 

 fine dividing line down the centre ; the front, or exterior, surface, 

 on the other hand, being without this division. In some genera, 

 as Barbula, the teeth are divided to the base, so that the above 

 structure cannot be traced. 



Acrocarpous mosses, with rare exceptions ; i.e., the 

 perichsetium is apical, forming a continuation of an ordinary stem 

 or branch, which may however appear lateral by subsequent 

 innovation below the perichaetium. 



Order VI. DICRANACEjE. 



Plants variable in size, dichotomously branched. Leaves 

 narrow, from subulate to broadly lanceolate, rarely ovate-oblong, 

 nerved nearly or quite to apex, areolation never wide, small, more 

 or less quadrate, rectangular or linear above, rectangular at base, 



