AND BUXBAUMIA. 343
Hymenostomum or Leptostomum. A membrane of this kind is certainly present in some species of Gymnostomum, and perhaps may be found in all those that really belong to that genus. It exists also in Weissia Templetoni, which so closely resembles Gymnostomum fasciculare as to be with difficulty distinguished from it, unless by the inspection of the peristomium ; and, in addition to the erect annular peristomium from which the character of the genus is taken, I have observed a similar membrane in Leptostomum itself. It seems even to be not an uncommon process or termination of the inner membrane, though it has been remarked only in some of its more obvious and persistent modifications. Thus the spongy membrane figured and described in the two published species of Calymperes, seems to be an analogous structure, 1 as is also the circular disk terminating the columella in several species oiSplach?ium ; [674 and perhaps even the tympanum of Polytrichum may be of similar origin.
But these characters of Leptostomum and Hymenostomum, though they do not appear to have been yet observed in any other mosses, may still perhaps be considered too minute for generic distinctions : and it must be admitted that were nothing to be obtained but the subdivision of an extensive natural genus it could not be necessary to have recourse to them. The divisions in question, however, are certainly not of that kind.
The weakest part indeed of Hedwig's system is its bringing together all those mosses that have a naked peri- stomium, and even including the greater part of them in the genus Gymnostomum ; while many of the species so associated are in real affinity much nearer to several other genera of the order having a simple or even a double peristomium.
1 The circular spongy membrane covering the mouth of the capsule certainly does not form an essential part of the character of Calymperes ; for, in the only species that I have examined, it is either entirely wanting, or firmly adheres to the inner surface of the operculum, along with which, also, a considerable por- tion of the columella separates. Nor has Swartz, who established the genus (in Spreng. Schracl. unci Link Jahrb. cler Gewach. vol. i, p. 1) even noticed this membrane in his description.
�� �
