214 Seward . — On Rachiopteris Williamsoni sp. nov ., 
similar to that in the larger canals of the fundamental tissue, 
and possibly representing the remnants of secretion. 
It remains for us to consider the probable affinities of the 
plant whose histological structure has been described in some 
detail. Its resemblances to Myeloxylon have already been 
referred to ; how far these should be looked upon as a proof 
of close relationship it is difficult to say. Possibly the acqui- 
sition of fresh material may help to bridge over such differ- 
ences as are at present apparent between the two fossils ; 
in any case it seems advisable to give expression to the well- 
marked characteristics of the present species by adopting 
a distinctive name. 
In Myeloxylon the collateral bundles and the position of the 
protoxylem are two important features ; these, as well as the 
nature of the canals, are strongly in favour of cycadean 
affinities. In Rachiopteris Williamsoni the bundles are 
concentric, there is a considerable development of xylem- 
parenchyma, the canals of the fundamental tissue do not 
appear to have the same distinct epithelial layer which is so 
clearly marked in Myeloxylon and recent Cycads ; and, finally, 
there are the smaller canals in the peripheral part of the 
phloem of each bundle. How far are these differences of 
taxonomic value ? 
The fact of a vascular bundle being of the collateral or 
concentric form is probably of no very great importance ; in 
recent Ferns we have the exceedingly common occurrence of 
collateral bundles in the finer branches of the fronds which 
have concentric bundles in their larger axes. In Rachiopteris 
Williamsoni some of the peripheral bundles approach much 
more closely to the collateral type than the larger and 
more distinctly concentric bundles. Haberlandt notes the 
occurrence of collateral bundles in the fronds of Marattia 
laxa and Angiopteris longifolia among the Marattiaceae 
and Mr. Brebner, who is at present engaged in a detailed 
examination of Marat tiaceous petioles, tells me he finds 
similar bundles in Marattia fraxinea. Granting this frequent 
1 Sitzgber. d. k. Akad. Wiss. Vienna, Abth. i. 1881, p. 7. 
