xvi FOSSIL ARCHEGONIATES 593 



gametophyte. This suggests the possibility of a derivation of 

 all of them from some type in which this two-sided apical cell 

 was permanent. Aneura and Metsgeria, among living genera, 

 have retained this condition, and in this respect are possibly 

 to be considered as representing the simplest type of the thallus. 

 The peculiar gemmae of the former, which may properly be 

 compared to the zoospores of Coleoch&te, strengthen this view. 



The peculiar chromatophores of the Anthocerotaceae, as well 

 as the structure of the sporophyte, make it conceivable that they 

 have originated independently, from forms lower than any exist- 

 ing Liverworts. It is quite possible, however, that the 

 Hepaticae and Anthocerotes represent two branches from a com- 

 mon stock, the multiple chromatophores of the true Hepaticae 

 being secondary, while Anthoceros has retained the primitive 

 single chromatophore, which has been replaced by the multiple 

 type in the other Archegoniates. 



Starting from the primitive type, found in Aneura or Metz- 

 geria, we have endeavoured to show that development proceeded 

 along two lines the Marchantiales and the Jungermanniales. 

 In the first one the differentiation consists mainly in the speciali- 

 sation of the tissues, while the gametophyte retains its strictly 

 thallose character; in the Jungermanniaceae it is rather in the 

 direction of the development of appendicular organs, while the 

 tissues remain nearly uniform. In both of these groups the 

 sporogophyte is comparatively simple, in strong contrast to 

 the Anthocerotes. The great preponderance of the foliose 

 Liverworts indicates that they are comparatively modern types, 

 which have adapted themselves to present conditions, and show 

 no indications of being connected directly with any higher 

 forms. 



Whether the Anthocerotes are considered to have been 

 derived from the lower Hepaticae, or whether they have origi- 

 nated independently of these, the differences are too great to 

 consider the group merely an order of the Hepaticae, coordinate 

 with the Marchantiales or Jungermanniales. Aside from the 

 peculiarities of the gametophyte, especially the primitive type 

 of chromatophore, the structure of the sporophyte of all the 

 Anthocerotes is radically different from that of the true He- 

 paticae, and forbids a direct association with any of them. 



Just as the simplest Jungermanniales may have served as 

 a starting-point for the two main lines of development in the 

 38 



