S8o MOSSES AND FERNS chap. 



The leaflets show no midrib, and are usually more or less fan- 

 shaped with radiating, dichotomously branched veins. 



A similar type of leaflet is found in some existing species 

 of Boirychium, e. g., B. lunaria, and also in species of Schizcea, 

 Trichomanes, Aneimia, and Adiantum. This type of venation 

 occurs in the cotyledon of most Ferns, and is probably to be 

 considered a more primitive one than the pinnate venation of 

 the typical Ferns. Two other characteristic types are the "Pe- 

 copteris" and the "Sphenopteris" types, which are represented 

 in many recent Ferns. The first, which dififers from the others 

 in having the pinnules sessile, by a broad base, is especially 

 common in the Cyatheacese, which includes most of the living 

 tree-Ferns. 



The netted venation seems to be the most recent type of all, 

 and Potonie states that it is first met with in Mesozoic fossils. 



The dichotomous branching of the leaf itself also seems to 

 be a primitive condition, and is relatively more common among 

 the Palaeozoic types than in those of the present. There are, 

 however, many examples among existing species, and it is the 

 usual form in the cotyledon. Gleichenia, Schizcea, Tricho- 

 manes, Matonia, Adiantum, are among the modern genera in 

 which this occurs. The Palseozoic Ferns also show not infre- 

 quently a condition intermediate between dichotomous and pin- 

 nate leaves. 



Another peculiarity of these ancient Ferns is the frequent 

 development of subsidiary pinnse between the ordinary ones. 

 These are rare in modern Ferns, but are known in a few cases, 

 e. g., Gleichenia gigantea, Hemitelia capensis. 



In the oldest fossils in which the sporangia have been de- 

 tected, these are confined to special leaves, or leaf-segments, as 

 they are in the living Ophioglossacese and Osmundaceae. 

 These fertile leaf-segments are quite destitute of a lamina, and 

 are completely covered by the sporangia. This condition of 

 things is an interesting confirmation of the view which con- 

 siders the Ophioglossacese as the most primitive existing type 

 of Ferns. This view holds that the primitive Fern type must 

 have developed the sporangial portion of the leaf before the 

 lamina appeared, a condition now known to exist in the curious 

 Ophioglossum simplex. 



The Devonian genus Archceopteris, for example, closely re- 

 3embles Botrychium, except that the fertile part of the leaf is 



