ch are characteristic of the wood of
the Pines and Firs (see fig. 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.
True Sponges (such as _Astrtoeospongia, Sphoerospongia_, &c.)
are not unknown; but by far the commonest representatives of
this sub-kingdom in the Devonian strata are _Stromatopora_ and
its allies. These singular organisms (fig. 79) are not only very
abundant in some of the Devonian limestones--both in the Old World
and the New--but they often attain very large dimensions. However
much they may differ in minor details, the general structure of
these bodies is that of numerous, concentrically-arranged, thin,
calcareous laminae, separated by narrow interspaces, which in turn
are crossed by numerous delicate vertical pillars, giving the whole
mass a cellular structure, and dividing it into innumerable minute
quadrangular compartments. Many of the Devonian _Stromatoporoe_
also exhibit on their surface the rounded openings of canals,
which can hardly have served any other purpose than that of
permitting the sea-water to gain ready access to every part of
the organism.

[Illustration: Fig. 79.--a, Par