( mess) 
XXIX.—On Rhetinangium arberi, a new genus of Cycadofilices from the Calciferous 
Sandstone Series. By W. T. Gordon, M.A., B.A., D.Sc., Lecturer in Paleon- 
tology, Edinburgh University. Communicated by Professor James GEIKIE, 
D.C.L., LL.D., ete. (With Three Plates.) 
(Read March 18, 1912. MS. received June 17, 1912. Issued separately December 28, 1912.) 
During the past few years probably no group of fossil plants has received more 
attention from paleobotanists than that of the Pteridospermez. Many fern-like 
impressions, derived from Carboniferous rocks, have proved to be members of this 
division of the vegetable kingdom ; while many more may ultimately be removed from 
the Filicales and included in the Pteridospermeze. Specimens, however, in which the 
internal structure is preserved, may, as a rule, be correctly referred to their respective 
class, but among those included in the latter group there is considerable diversity of 
organisation, and recently described genera have tended to increase the diversity of 
type rather than to indicate relationships among the forms already known. In fact we 
seem so far to have obtained only a very few examples of what must have been an 
exceedingly diversified plant family in late Paleeozoic times. 
Yet certain genera—Sutcliffia, Stenomyelon, etc.—exhibit anatomical characters 
which place them in an intermediate position between the polystelic Medullosew and 
the monostelic Lyginodendrex. These recently discovered forms, while helping to link 
up the extreme types, cannot be included in the same genus; their anatomy, indeed, 
seems to indicate several different lines of evolution within the group. 
The first petrifaction described from the Calciferous Sandstone rocks of Pettycur, 
namely, Heterangium grievu, was one of the simpler pteridospermous* forms, and the 
specimens were so abundant and so well preserved that the structure of this species is 
exceedingly well known. It has marked fern affinities, while cycadean characters are 
also present in the loose form of the secondary wood. No other Pteridosperm was 
obtained from the Pettycur rocks until about four years ago, when I collected what 
appeared to be a new species of Heterangiwm.t Subsequent investigation has shown that 
it differs markedly from Heterangiwm and also from all known genera of the Pterido- 
spermex, though it undoubtedly belongs to the Cycadofilices. I have been compelled, 
therefore, to make it the type of a new genus. In the choice of a name I have been 
guided by a characteristic feature of the anatomy, namely, the great development of 
what appear to be secretory sacs and ducts. At the same time Rhetunangiwn—the 
name adopted—suggests affinities with Heterangiwm. ‘The specific name was given in 
* Classed as a Pteridosperm on structural evidence only. Scorr, Studies in Fossil Botany, 1909. 
+ Heterangiwm arbert, Gordon: Thesis, On the Fossil Flora of the Pettycwr Limestone. University Library, 
Cambridge (1910), Edinburgh (1911). 
TRANS. ROY. SOC. EDIN., VOL. XLVIII. PART IV. (NO. 29). 118 
