218 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
cylinder seen alike in my fossil and in the Zygopteris Brongniarti and the Anachoro- 
pteris Decaisnii of M. Renault. In all these three plants the vessels form a distinct 
vascular cylinder which encloses a medullary tissue, reminding us of the simpler types 
of Lepidodendroid stems rather than of true ferns. Yet there seems to be no reason 
for doubting that M. Renault’s plants are true ferns ; and if so, my specimen corre- 
sponds with them too closely in all its essential features to be separated from Anachoro- 
pteris and Zygopteris. Assuming that this is the case, what are the lateral organs into 
which my stem obviously branched! My primary divergent bundles, indicated in all 
the figures by the letter c, went either to * secondary branches or to petioles. The 
analogies of M. Renault’s plants, as well as the general aspect of fig. 18, indicates that 
the latter was the case ; but if so, what were the smaller secondary bundles indicated 
by the letter cl 1 Springing directly from the central axis, and ascending through the 
cortex like the primary bundles, their direction seems to indicate that they also went 
to petioles, but to some of smaller size than those to which the larger and less numerous 
primary bundles were distributed. In his recently published memoir on Botryopteris 
forensis (Annales des Sciences Naturelles, 6 e serie, Bot., tome i. pi. 8. fig. 1) M. Renault 
has figured similar secondary bundles, given off, as in my plant, both from the central 
vascular axis and from the primary vascular petiolar bundle. The learned author does 
not hesitate to regard them as supplying rootlets. These moot points can scarcely be 
finally settled until we meet with additional examples of this curious stem. 
So distinctly characterized a stem requires an appropriate name. In my last memoir 
I endeavoured to avoid a needless multiplication of generic terms by grouping together 
a number of unidentified petioles under the common name of Bachiopteris . But we have 
now three examples of these petioles identified with stems, all of which latter exhibit 
a common type of internal organization, though their petioles display differences in the 
arrangement of their vascular tissues. But the latter fact constitutes no reason why 
the three plants should not be united in one common genus. In my previous Memoir* 
I pointed out that such differences of structure in the petioles did not prevent Adiantum 
trapeziforme and A. cuneatum from being placed in one genus, or Pteris umbrosa and 
P. aguilina in another. Neither should it prevent M. Renault’s Zygopteris Brongniarti, 
Anachoropteris Becaisnii, and my new plant from receiving a common generic name. 
But a new difficulty now springs up. Corda assigned each of his two generic terms, 
Zygopteris and Anachoropteris , to special forms of petioles. Hence if we select either of 
these names to represent the triad of plants in question it can only be done by giving 
an entirely new definition to an old generic name, and one, moreover, in the case of 
Zygopteris, that is still required for several petioles, such as Z. Lacattii and Z. bibrac- 
tiensis, of which the stems have not yet been found. Hence, though Zygopteris Brong- 
niarti must be included in the new genus, it seems undesirable to employ separate generic 
names for the three stems. 
Under these circumstances it appears to me to be wiser, for the present, to compre- 
* Phil. Trans, vol. 164. part 2, p. 676. 
