THE AMEBIC AN NATURALIST [Vol. XLVI 



evolution of the two groups are represented by existing genera. 

 It is hard to say which, of the two orders should be considered 

 the more primitive, as the lower members of each are of about 

 equal complexity, and can be derived equally well from some 

 form allied to SpJurroearpus. 



Sphcerocarpus has been associated most commonly with the 

 Ricciacea?, the lowest of the Marchantiales, but there are certain 

 genera of the Jungermanniales that in many ways show a close 

 resemblance to the Sphaerocarpaceoe, and make it almost certain 

 that there is a real relationship existing between them. These 

 similarities are found both in the character of the thallus and 

 reproductive organs, as well as in the early history of the embryo. 

 They may be only cases of parallel development, but it is quite 

 as likely that they are true homologies. Two genera, Petalo- 

 p/n/ltu»i and Fossomhronia, which have always been placed in 

 the Jungermanniales, are especially suggestive of a possible con- 

 nection with the Sphasroearpales, and it is by no means impos- 

 sible that it may turn out that these genera, and possibly some 

 others, should be removed from their association with the Junger- 

 manniales and transferred to the Sphaarocarpales. 



The Marchantiales 

 The Marchantiales constitute a very natural order, whose 

 simplest members, the Ricciacea?, are sometimes separated as a 

 distinct order. There does not seem to be any valid reason for 

 this, however, as the Ricciaceae are connected with the more 

 highly specialized Marehantiacea? by a number of intermediate 



The Marchantiales are comparatively few in number, probably 

 not more than three hundred species being known; but their 

 relatively large size and characteristic appearance make them 

 the most conspicuous of the liverworts, the common and wide- 

 spread Marchantia polymorpha being the most familiar liver- 

 wort to most students of botany. The dichotomously branched 

 tl'a Ins, with its elaborate systems of tissues, probably may be 

 said to represent the highest type of a strictly thallose plant. 



Within the Marchantiales are many interesting cases of 

 adaptation, and a very complete series of forms exists showing 

 the evolution of the elaborate and highly specialized thallus of 

 Marchantia and similar genera, from the much simpler type 



