VI] 



PTEROPSIDA 



81 



Several attempts have been recently made to determine 

 whether Archaeopteris was a member of the Primofilices or a 

 Pteridosperm. Kidston 1 and others think the latter the more 

 probable. We, however, agree with Johnson 2 that there is 

 absolutely no evidence that this very completely known type 

 (at any rate in the case of A. hibernica) bore seeds. It is much 

 more probably a Primofilix. The evidence of Rhacopteris, so far 

 as it goes, agrees with this conclusion. On the other hand at 

 least one Adiantites (Fig. 43) was a seed-bearing plant, but here 

 the seeds clearly arose by the metamor- 

 phosis of a segment of the frond. No 

 fructifications are yet known in the case 

 of Cardiopteris. 



Another very common type of frond 

 evolution is seen in Sphenopteridium (Figs. 

 34, p. 61, 35, p. 62 and 44), beautifully fore- 

 shadowed in the Psilophy ton flora by Pseudo- 

 sporochnus (Fig. 16, p. 34) and perhaps Ptilo- 

 phyton (Fig. 13, p. 32), and fully developed 

 in the Archaeopteris flora. 



Here the ends of large branches frequently 

 divide, usually dichotomously, to form a tuft 

 of very narrow forked, nerved segments 3 . 

 This type of frond persisted long after the 

 Devonian period. It is very common in Fig. 44. Sphenopterid- 

 Lower Carboniferous rocks, e.g. Sphenopteris iummoravicum,(Ett.), 



affinis (Fig. 45, p. 82) and S. bifida (Fig. j" rom the Lower Car - 



v *=> bomferous of Austria, 



46, p. 82) among many others, and it is Thalloid foliage (re- 



also met with in Upper Carboniferous times, duced to J nat. size). 



The genus Eremopteris simplv represents a After Stur, Culm- 



r J-* f- fxi.- , Flora (1875). 



somewhat peculiar modification of this type. 



Between Sphenopteridium and Sphenopteris it is not possible 

 to draw any really satisfactory line. The latter type possesses 

 broader and more rounded segments as a rule, with a pinnate 

 nervation. The term Sphenopteridium is in fact chiefly retained 



1 Kidston (1906), p. 435. 2 Johnson (1911 1 ). 



3 A similar type of leaf also occurs where the division of an Archaeopteris 

 or Rhacopteris type of pinnule is at its maximum. 



6 



