THE PENNSYLVANIAN PERIOD 595 



even then about as well differentiated from the horsetails and lycopods 

 as they are now, and so their ancestry is not revealed by close approach 

 to more primitive forms ; but, on the other hand, their relations to their 

 descendants seem to be shown by transitional forms, particularly by 

 an interesting group not yet well defined, the Cycadofilices, through 

 which they seem to have evolved into cycads, and perhaps into the 

 whole gymnospermous group, though this is an open question. 



The Cycadofilices (Pteridospermae). 1 — It has recently been ascer- 

 tained that not a few of the fern-like forms possessed structural features 

 which combine the characteristics of ferns and of cycads, and seem to 

 represent the line of evolution from ferns to cycads. They have hence 

 been called Cycadofilices by Potonie. 2 It has been further found that 

 some of these bore seeds, and such have been called Pteridospermce by 

 Oliver and Scott. 3 Of these, the best-known form is Lyginodendron, 

 illustrated by the excellent restoration of Scott and Allen (Fig. 278), 

 which exhibits the spiny stems and leaves, the highly dissected foliage, 

 the adventitious roots, and the general aspect. The leaves had well- 

 marked palisades (vertically elongated cells arranged side by side as 

 a protection against too intense sunlight, and hence significant of atmos- 

 pheric conditions), with the breathing pores (stomata) on the underside 

 of the leaf. "The whole structure is altogether comparable to that of 

 a fairly coriaceous fern-leaflet at the present day, and indicates 

 that the conditions to which the structure was adapted could not 

 have been fundamentally different from those which prevail in our 

 own epoch." 4 The limits of the group are not yet known, but it is 

 thought to include Lyginodendron, Heterangium, Megaloxylon, Clado- 

 doxylon, Calamopitys, Medullosa, and other genera founded on stem- 

 structure, and some at least of the species referred to Sphenopteris, 

 Alethopteris, Neuropteris, Odontopteris, Callipteris, Linopteris, Aneimites 

 (Adiantes), Callipteridium, and Lesleya, genera founded on fronds. 



1 Scott, Studies in Fossil Botany, Lectures X and XI, and Pres. Add. Roy. Misc. 

 Soc, Apr. 1895, Pt. 2, pp. 137-140. Also David White, Fossil Plants of the Group 

 Cycadofilices, Smithson. Misc. Coll., Vol. 47, Pt. 3, pp. 377, 390, 1905. 



2 Potonie, Lehrbuch die Pflanzenpaleontologie, p. 60, 1899. 



3 Proc. Roy. Soc, Vol. LXXI, 1903, p. 477, and Vol. LXXIII, 1904, p. 4; Phil. 

 Trans., Vol. 697, B, 1904, p. 193. As the bearing of seed places these forms among 

 the gymnosperm3 in the most extended sense of that term, it is doubtful whether the 

 term Pteridospermce can properly be retained, and the earlier term Cycadofilices is 

 given precedence here. 



4 Scott, Studies in Fossil Botany, p. 326. 



