2). The singular genus _Prototaxites_,
however, which occurs in an older portion of the Devonian series
than the above, is not in an absolutely unchallenged position.
By Principal Dawson it is regarded as the trunk of an ancient
_Conifer_--the most ancient known; but Mr Carruthers regards it
as more probably the stem of a gigantic sea-weed. The trunks
of _Prototaxites_ (fig. 78, A) vary from one to three feet in
diameter, and exhibit concentric rings of growth; but its woody
fibres have not hitherto been clearly demonstrated to possess discs.
Before leaving the Devonian vegetation, it may be mentioned that
the hornstone or chert so abundant in the Corniferous limestone
of North America has been shown to contain the remains of various
microscopic plants (_Diatoms_ and _Desmids_). We find also in
the same siliceous material the singular spherical bodies, with
radiating spines, which occur so abundantly in the chalk flints,
and which are termed _Xanthidia_. These may be regarded as probably
the spore-cases of the minute plants known as _Desmidioe_.
[Illustration: Fig. 78.--A, Trunk of _Prototaxites Logani_, eighteen
inches in diameter, as seen in the cliff near L'Anse Brehaut,
Gaspe; B, Two wood-cells showing spiral fibres and obscure pores,
highly magnified. Lower Devonian, Canada. (After Dawson)]
The Devonian _Protozoans_ have still to be fully investigated.
Pages:
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229