468 Mr. VauEntInE on the Development of the Theca, 
ever, appear to be few, as I have only detected three out of the very great 
number of species I have examined. In Sphagnum it is shaped somewhat like 
a button, having a very narrow neck, which is firmly embraced by the vagi- 
nula. This narrow neck is the only seta which exists in this genus, so that 
the theca is placed immediately on the vaginula. Muscologists, from not 
understanding the anatomy of this part of mosses, have denied the presence of 
a vaginula in the genus Sphagnum. Dr. Greville and Mr. Arnott, in their 
excellent memoir published in the Wernerian Transactions, have indeed 
maintained the existence of the vaginula; but they have described as such 
what ought not to be so considered. In this genus the calyptra, instead 
of dividing at the point where the sheath of mucus is secreted around the 
seta, is torn irregularly across the middle by the enlargement of the theca; 
and the scarious portion, which remains loose about the base of the theca, 
these observers have mistaken for the vaginula. The true vaginula, which is 
dilated and lentiform, to accommodate itself to the button-like termination 
of the seta, they call the receptacle, from not being aware of the internal 
structure. Another variety in the figure of the termination is in Schistostega 
pennata, where it is obovate, and the vaginula very much resembles in appear- 
ance the theca. But the most curious exception is in Dicranum flexuosum, 
in which the form is conical, but instead of being straight, is bent completely 
on itself. This structure can only take place by the second cell being de- 
veloped on the under surface of the first instead of on the upper, as happens 
in all other cases. The succeeding cell is placed transversely, and the rest 
assume the normal direction. 
To return to the progress of the development. A period of a month or more 
follows the separation of the calyptra without any further change taking place 
than the gradual elongation of the seta. In some instances, as Encalypta vul- 
garis, Tortula ruralis, and many more, three or four months are occupied by 
this process. The seta elongates by the addition of new matter at the apex, 
where it is always of a more delicate texture than nearer the base. The cells 
are also more crowded, less distinct in their outline, and have as yet no 
cavity. The further you examine from the apex, the more decidedly does the 
tissue become cellular, until it has arrived at maturity, when the cells are 
considerably elongated. After attaining a length, varying in each species 
