132 
NEW ARRANGEMENT OF 
tympanum, stretching across the interior of the theca, a little 
way above its base. The columella, therefore, taken with 
a view to the mature theca, may justly be described as ab- 
sent. The sporul(Z are in some cases almost pyramidal, 
with their sides convex and imperforated ; in others, sub- 
spherical and reticulated. 
DiFF. Char. The theca is entire, and sessile 07i the re- 
ceptacle. 
Veg. The stems are for the most part branched, and, 
from the influence of locality, or other causes, vary in length 
from one inch to above four feet. The leaves of none of 
the species are furnished with nerves ; in their form they 
are liable to the greatest variation, passing through ovato- 
obtuse, ovato-acuminate, lanceolate, into lanceolato-subu- 
late. They are either adpressed or recurved, or perhaps 
even secund, thus causing the extremities of the leaves to 
be sometimes obtuse and tumid, and sometimes attenuat- 
ed. A remarkable pale-yellowish or greenish-white colour, 
varied in some instances with a tinge of pink, is common 
to the whole genus. Their texture is also peculiar, being 
so tender as to render the apices liable to erosion, a circum- 
stance which is aided by the repeated changes of climate, 
naturally arising from their situation. This erosion has 
been made a ground for the constitution of species, by 
ScHWiEGiiiCHEN and others ; but it is a character which, 
with Dr Hooker, we consider of no importance, and not 
even constant to any one, and to which, therefore^ we can- 
not of course give our consent. The reticulation is no less 
curious, and is sufficient to distinguish this genus from 
nearly all others; it consists of oblong, longitudinal, flexuose, 
nearly regularly formed cellules, always interrupted by trans- 
verse lines. In one species {S, squarrosum) there are also 
