Mr. Brown on Lyellia, Leptostomum, and Buabaumia. 571 
Although there is but little resemblance in the structure of the 
peristomium among the different genera of Polytrichoidee, they may 
still be said essentially to agree in the function of this part: for in 
all of them the complete separation of the seeds is ensured by the 
smallness of the apertures for their discharge. It may be re- 
marked, that the necessity for this complete dispersion in Mosses 
seems to be inversely as the size of the seeds. For in those ge- 
nera of the order in which the capsule either bursts irregularly or 
has a naked mouth, the seeds are in general larger than in those | 
with a single, and still more manifestly than in those with a double, 
peristomium. And in conformity with this also, in Polytrichum 
undulatum and levigatum the tympanum is sooner ruptured or 
removed than in the other species of the genus. 
The result of this comparison of Polytrichum with Lyellia and 
Dawsonia, although it confirms the propriety of their approxima- 
tion, does not afford any clearly distinguishing mark for the very 
natural section of the order which these three genera form. In 
the mean time, however, it may be circumscribed, though not with 
absolute precision, by a combination of the more general charac- 
ters which have been now enumerated. 
LEPTOSTOMUM. 
In defining this genus, which was first proposed in my former 
paper on Mosses, I relied chiefly on the undivided annular pro- 
jection of the inner membrane of the capsule. I was induced 
to employ this modification of the peristomium as a character, 
though certainly far from being obvious, in finding it to exist in 
several mosses of the southern hemisphere, having a similar and 
peculiar habit ; and which, had it been neglected, I must have 
referred to Gymnostomum, with the greater number of whose spe- 
cies they have hardly any thing in common. 
VOL. XII. & E Mr. 
