and on the Sexes of Mosses. 471 



or columellai- 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 

 confhiuous at the base, with a delicate lining membrane, wiiich 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 vvhicii tiie columellar membrane has been detached, 

 and 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 Tortilla ; 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 tiie 

 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, M'hilst 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 Pterogonhim 

 declinatum ; and Bridel, the membranous ring of Hi/menosfomu?}!, 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 Pterogoiiuan intricatum, 

 another species of the same genus, arises from the thecal membrane ; and 

 Pterogoiiiinii gracl/e 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. 



