and on the Sexes of Mosses. 471 
or columellar membrane of this paper. The necessity of substituting this 
name will presently appear. To say that the outer peristome arises from the 
theca would give an incorrect idea both of its origin and connexion. It is 
continuous at the base, with a delicate lining membrane, which is very inti- 
mately attached to the theca. The existence of this lining membrane, which 
has hitherto escaped the notice of observers, may be proved by taking a 
portion of the theca from which the columellar membrane has been detached, 
aud carefully separating the peristome from above downwards, when the 
lining membrane will remain attached to the base. A very thin longitudinal 
section will also show the division of the theca itself into an external and 
internal layer. The former, when mature, is of a dense coriaceous or even 
horny texture; whilst the latter is of a loose spongy cellular tissue. The 
most favourable examples to prove this fact by dissection are found in the 
genus Tortula; but the Hypnums, a genus very remote from Tortula, are by 
no means unfavourable. The term lining or internal membrane ought pro- 
perly to be applied to this newly described layer; but, to prevent confusion, 
it appears desirable to abandon the use of this name altogether, and to supply 
its place with the term columellar membrane, designating the proper lining of 
the theca the thecal membrane. "The inner peristome is continuous with the 
columellar membrane, at the point where this last is attached to the inside of 
the stoma. These peristomes are not always formed. Some genera are alto- 
gether destitute of them, whilst others have only one, which, as far as my 
observations have gone, is always the external. Dr. Hooker, in the Linnean 
Transactions, vol. ix. p. 310, describes the single peristome of Pterogonium 
declinatum ; and Bridel, the membranous ring of Hymenostomum, as arising 
from the columellar membrane. With regard to the first plant I cannot give 
any positive evidence; but it seems probable that Dr. Hooker was mistaken, 
from not being aware of the presence of a thecal membrane. This probability 
is strengthened by the facts that the peristome of Pterogonium intricatum, 
another species of the same genus, arises from the thecal membrane; and 
Pterogonium gracile has actually a double peristome. As to the origin of the 
peristome in Hymenostomum I can speak with greater certainty, as 1 have 
frequently dissected away the columellar membrane entire ; and the peristome 
was in every case left attached to the thecal membrane. This latter peristome, 
