CYCADOFILICES FROM THE CALCIFEROUS SANDSTONE SERIES. 821 
DiaGnosis. 
Rhetmangium, gen. nov. 
The characters of this new genus are, meanwhile, those of the only recorded species, 
R. arbern. 
Rhetinangvum arberi, sp. nov. 
Stem 2 cm. in diameter, circular in transverse section and surrounded by spirally 
developed leaves. Central vascular axis protostelic, consisting of anastomosing 
groups of tracheides in a parenchymatous ground tissue with secretory ducts. Proto- 
xylem exarch, and with scalariform or sub-spiral thickening. Xylem (primary or 
secondary) of long, reticulately thickened, porose tracheides. Medullary rays broad and 
high. Phloem and inner cortex with many secretory cells and ducts. Outer cortex 
of thick-walled parenchyma. Hypodermal zone of sclerotic anastomosing fibres 
associated with elongated secretory ducts. 
Petiole-trace an aggregate of several peripheral xylem groups loosely attached 
together to form a corrugated band. Protoxylems abaxial. Leaf-bases without sclerotic 
hypoderma, but outer cortex enormously expanded. Diameter of petiole beyond base 
not abnormally large compared with stem but showing reappearance of sclerotic outer 
cortex. Roots tetrarch ; secondary wood well developed. 
Foliage and fructifications unknown. 
Localitues.—Pettycur, Fife, Scotland, and Edrom, Berwickshire, Scotland. 
Horizon.—Calciferous Sandstone Series (= Culm). 
AFFINITIES AND GENERAL CONSIDERATIONS. 
The primary wood in Rhetinangium is similar in its general structure to the xylem 
cylinder in certain living protostelic ferns, and, in particular, since the xylem is exarch, 
to that of Lygodium. At the same time the type of the secondary wood, with its 
numerous broad and high medullary rays, is distinctly cycadean. The structure of 
the axis thus places Rhetenangiwm in an intermediate position between ferns and cycads. 
In any case, the new genus possesses all the characters necessary for its inclusion in the 
group of the Cycadofilices. 
Although its fructification is unknown, it is quite admissible to refer this new plant 
to the Pteridospermex on anatomical grounds alone, but its structure does not indicate 
any very obvious relationship with other members of that group. The genus, however, 
is not a perfectly isolated one, since there are marked resemblances between its anatomy 
and that of certain other forms. The primary xylem recalls the corresponding tissue 
in Megaloxylon, Heterangium, Medullosa, and in a less degree that in Suteliffia. 
The exarch nature of the xylem, however, at once separates this new type from Heteran- 
guum and Medullosa (which latter, as far as its vascular axis is concerned, is really a 
polystelic Heterangium). With Megaloxylon, on the other hand, the affinities are 
much closer: apart from the difference in size, indeed, the primary wood in both is 
TRANS. ROY. SOC. EDIN., VOL, XLVIIT. PART IV. (NO. 29). 119 
