568 Mr. Brown on Lyellia, Leptostomum, and Burbaumia. 
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 theinternal structure, which 
I believe is not the case, it seems hardly sufficient to justify me 
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 Lyellia 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- 
trichum the only observation that I have met with occurs in Eng- 
lish Botany, where, in the account of Polytrichum subrotundum 
(plate 1624), it is stated that Mr. James D. Sowerby has disco- 
vered, in this species as well as in Polytrichum undulatum, 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 lamelle. 
Besides this transverse annular projection there are in the inner 
membrane of all the species of Polytrichum that I have examined, 
P. undu- 
