568 Mr. Brown on Lyellia, Lcptostomutn, and BiLvbmimia, 



considered as a distinct genus by Mons. de Beauvois. But un- 

 less this difference of external form should be found connected 

 with others, either in the habit or in the internal structure, which 

 I believe is not the case, it seems hardly sufficient to justify the 

 subdivision of so natural a genus. 



The symmetrical quadrangular capsule, admitting of a regular 

 cylindrical inner membrane, is a character of inferior importance 

 to the plano-convex or dimidiate capsule, which almost neces- 

 sarily implies, and in Dawsonia and LyeUia at least is found to be 

 connected with, a corresponding irregularity in the figure of the 

 cavity; and hence I have introduced this remarkable form into 

 the characters of both these genera. 



On the structure of the inner membrane of the capsule in Poly- 

 frlchum the only observation that I have met with occurs in Eng- 

 lish Botany, where, in the account of Polytrichum subrotiindum 

 (plate 1624), it is stated that Mr. James D. Sowerby has disco- 

 vered, in this species as well as in Polytrichum nndulatum, a real 

 membranaceous peristomium within the teeth, which, according 

 to Sir James Smith, " gives quite a new idea of the generic cha- 

 racter." 



This inner peristomium, if it may be so termed, which is well 

 represented in the figure referred to, and consists of a horizontal 

 projection of the inner membrane immediately below its apex, 

 will, I believe, be found to exist in all the species of the genus, 

 and in some cases reduces the aperture of the inner capsule to 

 half the size of the outer at the origin of the teeth. It is always, 

 however, quite entire, and, according to my observations, is formed 

 of a doubling of the inner membrane, with a loose cellular or 

 rather spongy substance interposed between the lamellae. 



Besides this transverse annular projection there are in the inner 

 membrane of all the species oi Polytrichum that I have examined, 



P. undu- 



