deposits as the Old Red Sandstone proper and the Gaspe series of North America, and few traces of plant-life occur in the strictly marine sediments.  Apart from numerous remains, mostly of a problematical nature, referred to the comprehensive group of the Sea-weeds, a large number of Ferns have now been recognised, some being, of the ordinary plant-like type (Pecopteris, Neuropteris, Alethopteris, Sphenopteris, &c.), whilst others belong to the gigantic group of the “Tree-ferns” (Psaronius, Caulopteris, &c.) Besides these there is an abundant development of the singular extinct types of the Lepidodendroids, the Sigillarioids, and the Calamites, all of which attained their maximum in the Carboniferous.  Of these, the Lepidodendra may be regarded as gigantic, tree-like Club-mosses (Lycopodiaceoe); the Calamites are equally gigantic Horse-tails (Equisetaceoe); and the Sigillarioids, equally huge in size, in some respects hold a position intermediate between the Club-mosses and the Pines (Conifers).  The Devonian rocks have also yielded traces of many other plants (such as Annularia, Asterophyllites, Cardiocarpon, &c.), which acquire a greater pre-dominance in the Carboniferous period, and which will be spoken of in discussing the structure of the plants of the Coal-measures.  Upon the whole, the one plant which may be considered as specially characteristic of the Devonian (though not confined to this series) is the Psilophyton (fig. 77) of Dr Dawson.  These singular plants have slender branching stems, with sparse needle-shaped leaves, the young stems being at first coiled up, crosier-fashion, like the young fronds of ferns, whilst the old branches carry numerous spore-cases.  The stems and branches seem to have attained a height of two or three feet; and they sprang from prostrate “root-stocks” or creeping stems.  Upon the whole, Principal Dawson is disposed to regard Psilophyton as a “generalised type” of plants intermediate between the Ferns and the Club-mosses.  Lastly, the Devonian deposits have yielded the remains of the first actual trees with which we are as yet acquainted.  About the nature of some of these (Ormoxylon and Dadoxylon) no doubt can be entertained, since their trunks not only show the concentric rings of growth characteristic of exogenous trees in general, but their woody tissue exhibits under the microscope the “discs” which 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