814 W. T. GORDON ON RHETINANGIUM ARBERI, A NEW GENUS OF 
honour of Dr E. A. Neweit ArBer, to whom I was much indebted for his kindly 
criticism and advice while conducting my research work at Cambridge. 
DESCRIPTION OF THE SPECIMENS. 
As already stated, the first specimen was obtained about four years ago, and the 
structure determined as far as possible; but a second example was discovered more 
recently at the same locality; while a third, which may be specifically identical with 
these two, was collected by Dr Kipsron on the bank of the Whiteadder Water near 
Edrom, Berwickshire. All three specimens were derived from rocks of Lower Carboni- 
ferous age and probably from similar horizons. The Berwickshire stem was obtained 
several years ago, but was so indifferently petrified that it could not be described. The 
following description has therefore been based on the remains from Pettycur, which are, 
on the whole, well preserved, though not uniformly so. This variable preservation is 
very noticeable in the shorter of the two specimens, and, unfortunately, the best portion 
lay exposed on the surface of the limestone block, with the result that it had been 
partially weathered away. When followed further into the mass this stem was poorly 
petrified and finally disappeared. Its total length did not exceed 5} inches. 
The second example was nearly twice as long (10 inches), and it passed completely 
through the block in which it was embedded. From both specimens series of transverse 
and longitudinal sections have been prepared. These number about 120, and a very 
fair idea of the anatomy of the plant has been obtained from them. 
Several petioles were also discovered, either in organic continuity with the stems or 
closely associated with them, while a root was obtained attached to one of the specimens. 
GENERAL STRUCTURE. 
A transverse section of the stem is represented in Pl. I. fig. 1, and, although the 
central xylem cylinder is not particularly well preserved in this specimen, a good 
general idea of the anatomy may be obtained by an examination of the figure. The 
actual measurements of the stem are 2°5 em. x1'd cm., but the flattening is entirely 
due to compression prior to fossilisation. This is proved by the fact that the cortical 
tissues at the ends of the longer diameter are crushed together. The stem in the living 
plant would therefore be circular in section and probably about 2 cm. in diameter. 
In the centre of the specimen is the vascular axis, which consists of an internal zone 
of primary wood (#'), and an external one of secondary wood (x), Surrounding the 
xylem cylinder is the phloem and inner cortical tissues, while enclosing the whole is a 
thick layer of outer cortex with a well-marked hypodermal zone of anastomosing 
sclerotic strands. A general resemblance to the stem of Heterangium will at once be 
noticed, but a more detailed examination will disclose several differences between 
Rhetinangivm and that genus. 
