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, Part of the under surface of Stromatopora tuberculata, showing the wrinkled basement membrane and the openings of water-canals, of the natural size; b, Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, magnified to show the internal structure.  Corniferous Limestone, Canada. (Original.)]

[Illustration:  Fig. 80.—­Cystiphyllum vesiculosum, showing a succession of cups produces by budding from the original coral.  Of the natural size.  Devonian, America and Europe. (Original.)]

[Illustration:  Fig. 81—­Zaphrentis cornicula, of the natural size.  Devonian, America. (Original.)]

[Illustration:  Fig. 82—­Heliophyllum exiguum, viewed from in front and behind.  Of the natural size.  Devonian, Canada. (Original.)]

[Illustration:  Fig. 83.—­Portion of a mass of Crepidophyllum Archiaci, of the natural size.  Hamilton Formation, Canada. (After Billings.)]