194 
F. Cavers. 
growth of a thallus-lobe, but are sometimes seated on the dorsal 
side of the thallus. The male receptacle is at first sessile on the 
thallus, but is sometimes raised later on a very short stalk, which 
has, however, no rhizoid-furrow. The receptacles of both kinds are 
of the “ Composite ” type. In the male receptacle the antheridia 
are developed in centrifugal order on the upper surface; the oldest 
antheridia occupy the centre of the receptacle, while the marginal 
tissue remains embryonic and gives rise to new antheridia, though 
there is apparently no definite arrangement in rows answering to a 
number of growing-points. The under side of the male receptacle 
bears scales and rhizoids. Immediately below and in front of the 
developing receptacle there appears an adventitious branch, which 
continues the forward growth of the thallus, so that the receptacle 
(which arises in the first instance as a terminal, i.e., apical, structure) 
is shifted on to the dorsal side of the thallus. This continuation- 
branch then proceeds, either at once or after giving rise to a tract 
of thallus-tissue, to develop a female receptacle. The latter shows 
from four to ten groups of archegonia, each group being surrounded 
by an involucre and containing as many as twelve archegonia ; the 
groups are separated by lobes bearing scales and rhizoids, which 
occur also on the lower part of each involucre. The ripe sporo- 
gonium strongly resembles that of the Corsiniaceae, having a spherical 
capsule and a short “ foot ”; the capsule-wall consists of a single 
layer of unthickened cells—except at the apex where there is a cap 
composed of several layers, some of which have rudimentary 
thickening-fibres; the sterile cells mixed with the spores are short 
and retain their chlorophyll, and are either unthickened or show 
rudimentary annular or spiral fibres; the spores remain united in 
tetrads when ripe; the seta remains undeveloped, the capsule is not 
exserted from the involucre, and the spores escape merely by the 
irregular brealdng-up of the capsule-wall. 
We may regard Monoselenium as a reduced form, the reduction 
extending to all the organs but being exhibited by the different 
organs in varying degrees. The thallus resembles that of Monoclea 
in having no trace of an air-chamber zone, but the ventral scales, 
instead of being reduced to unicellular or filamentous gum-papillae, 
resemble rather those of Dumortiera or Cyathodium —which, as we 
have seen, are certainly reduced forms. In the structure of the 
sporogonium, Monoselenium forms a perfect connecting-link between 
the genera Corsinia and Boschia ; sterile cells resembling those of 
Corsinia, Sphcerocarpus, and Riella, occur in the same capsule with 
