VI] 



PTEROPSIDA 



79 



is first the Archaeopteris type of leaf. This arose from the meta- 

 morphosis of the branches of the nth. order in a system which 

 was at least bipinnate, if not more compound still. Each 

 alternate branchlet was flattened out either into a sterile wedge- 

 shaped leaf, with a radiating nervation, or metamorphosed into 

 one or more sporangia. Every stage is clearly seen in Devonian 

 species of Archaeopteris (Figs. 28-31, pp. 56-59). Further 

 modifications of the wedge-shaped entire, primitive type of leaf 

 soon set in. As in Psygmophyllum, these leaves tend to become 

 lobed or segmented longitudinally. 



In A. Archetypus, Schmalh. (Fig. 29, p. 57) 

 and A. Roger si, Daws., the leaflet is primi- 

 tive, large and undivided. We next pass on 

 to types such as A. hibernica, (Forbes) (Fig. 28, 

 p. 56) and A. Roemeriana, (Goepp.) in which 

 the leaf tends to become unsymmetrical and 

 more or less toothed or lobed, and finally we 

 reach such types as A. fimbriata, Nath. (Fig. 30, 

 p. 58) and A. fissilis, Schmalh. (Fig. 31, p. 59) 

 in which it is divided nearly to the base, longi- 

 tudinally, into very narrow segments. A still 

 further elaboration of this type by splitting 

 would be indistinguishable from Spheno- 

 pteris. 



Further, other later genera, especially cha- 

 racteristic of the Lower Carboniferous, such 

 as Rhacopteris' (Figs. 42, and 33, p. 61), 

 Adiantites (Fig. 43, p. 80) and Cardiopteris, have 

 leaves essentially similar to Archaeopteris and 

 with a radiating nervation. Further, here also 

 the tendency to longitudinal splitting is marked 

 at least in the two genera first' named. Such 

 types are in fact chiefly distinguished from 

 Archaeopteris by the shape of the leaf and its 

 segments — by some small peculiarity of its 

 symmetry. They are all, however, obviously derived from the 

 Archaeopteris type of leaflet. 



1 In this genus the frond also dichotomises. 



Fig. 42. Rhaco- 

 pteris paniculi- 

 fera, Stur, from 

 the Lower Car- 

 boniferous of 

 Austria. Fertile 

 frond (reduced to 

 J nat. size). After 

 Stur. Culm-Flora 

 (1875). 



