THE GENERA OF MOSSES. 127 
DiFF. CiiAR. The theca is Jbur-valved ; valves coher- 
ing at their apices by means of the persistent operculum. 
Vegetation. The stems (caules) are branched, seldom 
more than half an inch long in two species, while in A. ni^ 
valis they reach sometimes five or six inches. The leaves 
are ovate, and, from being more or less suddenly acumi- 
nated, pass into lanceolate ; a dark-brown, or even almost 
black, colour is common to them all, yet they are some- 
times inclined to yellow. Two of the species have secund 
leaves, while a third possesses that character only at the 
summits of the branches : the other, A. alpina, has all the 
leaves erect, or erecto-patent. The order, however, of the 
distribution of the nerves of the leaves seems to be inverted, 
A. alpina and rupestris are entirely destitute of a nerve ; 
A. Rothii has a nerve in all, except the perichsetial leaves ; 
while A. nivalis, in both the perichsetial and cauline leaves, 
is furnished with a very strong one. 
Observations. The truly sessile theca seems to ally 
these plants most strongly to Sphagnum, but that is the 
only point in which they can be said to bear any resemblance 
to each other. The four-valved theca is a character by which 
the species of this curious genus may be separated from all 
other known mosses ; and notwithstanding the central colu- 
mella, the robust habit, the sporulae destitute of elateres or 
spiral filaments, and leaves (in two species) furnished with 
nerves, one can scarcely wonder at some of the older Bo- 
tanists regarding them as Jungermannioe. In a natural 
arrangement, therefore, they form an admirable link be- 
tween the Hepaticje and the Musci. 
What we term Theca is very different from what was 
here so called by Hedwig. According to him, our theca 
were teeth, and our apophysis his theca : thus directly at 
