470 3Ir. ^^VLENTINE 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 sepaiation) 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 liave observed in one instance, the Hymenostominn of 

 BroM'n, the columella to separate below as well as above the point of con- 

 nexion with the columellar membrane. The opercular membrane, when 

 mature, either remains attached to the columella, falls with the operculum, 

 or (in the genus Polytrickum) shrivels from below upwards, and remains 

 attached 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 D'lphi/schim ; whilst in one 

 instance, Buxbatimia, it is double ; the external splitting into cilice, 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. Tliis 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 eitlier 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 lieen 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. 



