470 Mr. VALENTINE on the Development of the Theca, 
the inner surface of which it is reflected upwards to the stoma. The mem- 
brane is attached to the stoma all round, frequently by a distinct process; 
and after forming this attachment, it passes horizontally inwards, and becomes 
again continuous with the columella at its apex. Until about the period of 
maturity, or a little earlier, the columella is continuous from the base of the 
theca up to the arch of the operculum, when a transverse line (indicating a 
tendency to separation) appears above the point of its connexion with the 
columellar membrane. Most commonly this separation does actually take 
place, and the upper portion falls with the operculum. This portion was first 
described and named, very appropriately, by Greville and Arnott, the opercu- 
lar membrane. I have observed in one instance, the Hymenostomum of 
Brown, the columella to separate below as well as above the point of con- 
1exion with the columellar membrane. The opercular membrane, when 
nature, either remains attached to the columella, falls with the operculum, 
or (in the genus Polytrichum) shrivels from below upwards, and remains 
ittached to the apices of the teeth of the peristome in the form of a horizontal 
membrane or tympanum. 
In an early stage the inner layer of the operculum separates in the form of 
a distinct membrane, which, ultimately dividing longitudinally into a definite 
number of processes or teeth, forms the peristome. In some rare instances 
this membrane never breaks up into teeth, as in Diphyscium ; whilst in one 
instance, Burbaumia, it is double; the external splitting into ciliz, and the 
internal remaining entire. At the same time that this membrane is formed 
from the operculum, the opercular membrane forms another, immediately 
within the first, by a separation of its exterior series of cells. This also, more 
or less, divides longitudinally into a determinate number of teeth, thus forming 
the inner peristome. The number of teeth forming each of these peristomes 
has been ascertained by muscologists to be either four, or a multiple of that 
number*. ‘The outer peristome is universally considered to arise from the 
theca itself; whilst the inner is believed to arise from the internal membrane, 
* Mr. Brown appears to have been the first to point out the mode of ascertaining the true number 
of the teeth. This great botanist reduces the number of the outer series in most instances to thirty- 
two. Vide Linnean Transactions, vol. xii. p.577., where may be found some excellent observations 
on this subject. 
