60 



BRITISH EOCENE FLORA. 



seems more closely to agree with them in this, as well as in the imdulation and frequent 

 crenation of the margin of the longer pinnules. 



In spite of so many points of resemblance the specific difference between the fossil and 

 its existing ally is extremely well marked. The fossil form appears to have possessed 

 a longer and more repeatedly branching stem. In the recent form the pinnae are 

 dichotomous, and, as I interpret the fossil species, it also was dichotomous, for the hook 

 or short tendril at the base of the single pinna represents the aborted second pinna 

 converted into this singular form. This highly specialised condition may have been 

 induced by the comparative length and weakness of the rachis, the want of rigidity in 

 which is indicated in the fossil by the somewhat sinuous midribs of the pinnae. 



Fig. 28. — Habit of Gleichenia Ilantonensis restored. 



The conversion of these pinnae into tendril-like supports appears to have been 

 compensated by the former development of the now rudimentary buds, always present 

 between the forking of the stem, into a third pinna or smaller pair of pinnae.^ With the 

 disappearance of the tendrils in the existing plant this central bud has ceased to expand, 

 or exceptionally becomes a barren adiantoid bract. PI. VI, fig. 3, may represent one of 

 the simple pinnae from the base of the forked branches, characteristic of the section of 

 G. dicltotoma. The position of these in the existing plant is shown in fig. 2S. 



Remains of this fern were formerly so abundant at Bournemouth that it is to be 

 ^ The restoration shows a pair of terminal pinnae, taken from Plate X, fig. 2. 



