Phytogeny of Marchantiales. 181 
be more appropriately discussed later. The study of the geo¬ 
graphical distribution of the existing Hepaticas leads to the 
conclusion that the thalloid forms at any rate (the Marchantiales 
and probably also the lower Jungermanniales) are ancient forms. 
As Campbell (9) has pointed out, the scarcity of fossil Bryophytes 
may be partly attributed to the delicate and perishable nature of 
their tissues. At present no conclusions as to the antiquity of the 
different groups of Bryophyta can possibly be drawn from the very 
scanty and doubtful Palaeozoic remains that have been referred to 
provisional genera like Marchantites and Muscites ; see Scott’s 
summary (58). The earliest known undoubted fossil Bryophyte is 
a Marchantia- like form ( Marchantites ) found at least as far back as 
the Jurassic. Judging from the habitats of the existing forms, the 
only liverworts that might be expected to occur in the Palaeozoic 
rocks are the shade-loving, hygrophilous, and epiphytic species, 
belonging chiefly to the leafy Jungermanniales. The existing 
Marchantiales are much more completely adapted to land life, and 
to life in open and exposed situations, than are the Jungermanniales. 
Xerophilous forms occur in all the groups of Hepaticae, but the 
proportion of such forms is largest in the Marchantiales. 
The habitats of the Marchantiales are also of interest in 
considering the question whether the existing members of this 
group really form an ascending series starting with Ricciaceae, or 
whether the Ricciaceae, and perhaps also the Corsiniaceae, are to 
be regarded, on the other hand, as reduced forms. The only genus 
of Marchantiales which clearly shows reduction in both gametophyte 
and sporophyte is Cyathodium, and as Lang (35) has pointed out, 
this genus “ is to be regarded as primarily a form adapted to shade 
conditions rather than as a hygrophyte.” In this respect Cyathodium 
forms a contrast to the markedly hygrophilous genera Dumortiera 
and Monoclea, in which there is no evident reduction in the sporophyte 
though the thallus shows extensive or even complete degeneration 
of the air-chamber layer. In the Ricciaceas we have a number of 
hygrophilous species ( Riccia crystallina and its allies), and in two 
cases a terrestrial species has a free-floating aquatic form, either 
swimming ( R. natans, the aquatic form of R. lutescens) or submerged 
(R. fluitans, the aquatic form of R. canaliculata), but the only 
liverwort which fruits under aquatic conditions is Riella, and Riella 
is almost certainly derived from a Spheerocarpus- like form by 
adaptation to submerged aquatic life, and cannot be regarded as a 
primitively aquatic type. 
