THE GENERA OF MOSSES. 
47 
portion of which, is attached all round to the sporular bag 
at the margin of the theca. That part of this membrane 
which unites the lining of the theca to w^hat is usually 
called the summit of the columella, is in this genus com- 
monly of a strong nature, — so much so, as to induce Mr 
Brown, and also ourselves at one time, to conclude that 
G. microstomum * possessed a horizontal membrane actual- 
ly arising from the wall of the theca. This, however, is not 
the case : but even supposing it were so, the difficulty of 
examination is so great, and the membrane itself so eva- 
nescent, this character alone would be but of little use^ 
though certainly sufficient to remove any plant from the 
true Gymnostoma. 
With the membrane which covers the sporular bag, the 
peristome of Leptostomum must not be confounded. In 
that genus, Dr Hooker remarks, that the peristome is very 
near that of Diphysclum ; and we add, also, to the internal 
peristome of Bua^haumia and Ptychostomum, (HoRNscH.)j; 
but Leptostomum and Ptychostomum we think more natu- 
rally allied to Bryum. 
We have already mentioned that some species possess 
an annulus. Dr Hooker, in his " Musci Exotici,'' has 
figured two, G. julaceum and involutum, Weber and 
MoHR (Handbuch, p. 86.) first pointed out this part in 
G, tenue, — " Arte solventes operculum, annulum s. fim- 
briam, cujus nec Hedw. nec Schrad. mentionem fecerunt, 
observavimus." This was soon afterwards also observed by 
RoHLiNG. To these three must be added, G. trichodes 
• Mr Brown constitutes of this plant his genus Hymenostomum, with 
the character—" Stoma edentulum, clausum epiphragmate (e membrana 
exteriore orto), disco tenuissimo (a columello libero) mox rupto et evanido j 
limto persistente, horizontal! indiviso."*».Linn. Trans, v. 12. p. 572. 
2 
