560 Bancroft.—A Contribution to our 
species of the genus realize this phase at a lower level of petiolar develop¬ 
ment—that is, at the base of the petiole instead of in an ultimate branch. 
They may thus be regarded as further from the ancestral type in this 
respect than B. forensis ; also their simplicity cannot be due to primi¬ 
tiveness. 
Again, since the traces of B. antiqua and R. cylindrica are clearly 
simpler members of the same series as the tridentate forms, 1 it may be 
argued that further simplicity is due to further reduction ; if this be so, the 
oldest species of the series must possess the most reduced trace. 
It has been shown, however, that the foliar traces of B. antiqua , 
R. cylindrical and B. ramosa and hirsuta form a connected progressive 
series which may be derived from an ancestral type of trace ; and, in the 
absence of advancing intermediate stages connecting the simple tridentate 
traces with that of B. forensis , it may be advisable to consider this species 
as being somewhat removed from the earlier types. 
In stem structure also, B. forensis differs from the British types, for the 
solid protostele is definitely exarch. 2 
The plant is known to have possessed a leaf with thick, dichotomously 
branched veins and small fleshy pinnules—the only example of an ordinary 
leaf yet known amongst the species of Botryopteris. 
The sporangia, again, seem to isolate B. forensis from the British 
species. They have been described as possessing a two-layered wall, 3 the 
inner layer being often distinguishable only as a thin membrane. Accord¬ 
ing to Oliver, 4 * some specimens referred to the provisional genus Tracheo- 
theca 5 may perhaps have belonged to B. forensis ; their walls are lined with 
a delicate tracheal layer unlike anything observed in the earlier species. Its 
presence, of course, may be merely the result of environmental influences. 6 
It is concluded that Rachiopteris cylindrica is allied to Botryopteris 
antiqua , B. ramosa , and B. hirsutai the four species forming a group, with 
which B.forensis is not very closely related. This conclusion suggests that 
the desirability ofincludin % Rachiopteris cylindrica in the genus Botryopteris 
must be determined by the desirability of retaining there its three related 
types. 
3. Theoretical Considerations. 
Rachiopteris cylindrica presents several points of theoretical interest, 
two of which will be briefly discussed. 
1 Kidston (’ 08 ), p. 364. 2 Renault (’ 83 ), p. 104; Seward (’ 10 ), p. 437. 
3 Renault (’ 96 ), pp. 53 and 54. 
4 Oliver, F. W. : A Vascular Sporangium. New Phyt., vol. 1, 1902, p. 60. 
5 Oliver, F. W.; On the Structure and Affinities of Stephanos per mum , Brongniart, a Genus of 
Fossil Gymnosperm Seeds. Trans. Linn. Soc., Bot., vol. 6, 1904, p. 361. See foot-note, p. 395. 
6 Seward (TO), p. 443. 
