PTERIDOPHYTA. 1503 



leaves, the existing tropical genus Lygodium occurs in Europe from 

 the Cretaceous to the Miocene, and also in the Laramie beds of the 

 United States. The third family — Gleicheniacetz — has the sessile 

 sporangia borne in naked sori upon the dorsal surface of ordinary 

 leaves, each sporangium having a complete transverse ring, and 

 bursting with a longitudinal slit ; while the stem forms a creeping 

 rhizome. The existing tropical genus Gleichenia is represented by 

 closely allied forms in the Cretaceous of Greenland, the Rajmahal 

 group of the Indian Gondwanas, and in the Lias of Verona, some 

 of which have been separated as Gleichenites. Didymosorus, from 

 the Cretaceous of Greenland and Saxony, with two sori on each 

 leaflet, connects the preceding with Mertensia. The latter, which 

 has two rows of sori on each leaflet, and is now confined to the 

 southern hemisphere, occurs in the Cretaceous of Greenland and 

 Hungary. The Hymenophyllacece, or Film-ferns, in which the sessile 

 sori are covered by an indusium, are represented by one species in 

 the Upper Eocene of Provence ; while Schimper has referred to the 

 type genus Hynmwphyllum a Carboniferous fern. It is probable 

 that some of the Sphenopteridece. are more or less closely related to 

 this family. The fifth family — Cyathecacece — which includes the 

 tropical Tree-ferns, is of more palaeontological importance ; it is 

 characterised by the stalked sporangia, situated in closely packed 

 sori, which may be naked or covered. Of Tree-ferns, in which 

 there is a tall stem, often covered with roots, and crowned with a 

 rosette of leaves, the existing Alsophila is represented in the Lower 

 Eocene of Sezanne, which also contains other allied forms. The Car- 

 boniferous Choroniopteris should perhaps find a place here. Of smaller 

 forms, the genus Onodea (in which Schimper includes Struthiopteris) 

 is represented in the Laramie Cretaceous and the Miocene of the 

 United States, as well as in the Eocene of the Isle of Mull, by the 

 existing 0. sensibilis of North America ; thus offering a remarkable 

 instance of the persistence of a specific type. The Mull form was 

 originally described as Filiates. The Laramie series also yields an 

 extinct species of this genus. These ferns, like the following, have 

 the sporangia borne on specialised leaves. Dicksonia, which is 

 mainly tropical and often tree-like, occurs in the European Keuper, 

 and more abundantly in the Jurassic, and is also found in the 

 Indian Rajmahal beds. Finally, Thrysopteris, now known only 

 by a single species from Juan Fernandez, is also very common 

 in the European and Siberian Jurassic (fig. 1368), where it has 

 been described as Coniopteris and Tympa?iophora. In addition 

 to the special fertile leaves, a few sori are borne at the base of 

 the ordinary leaves, as shown in the figure. 



Lastly, we come to the extensive family Polypodiacetz, which 

 includes the greater number of European ferns, and is divided 



