These beds are the equivalents of the Middle Coal-measures, or Productive Coal-measures of Pennsylvania, Ohio, &c., and of the coal-formation proper of various European countries. Very many of the species are common to Nova Scotia and Pennsylvania; but in proceeding westward the number of identical species seems to diminish. (3) _The Millstone Grit Sub-Flora_: In this formation the abundance of plants and the number of species are greatly diminished.[FN] Trunks of coniferous trees of the species _Dadoxylon Acadianum_, having wide wood-cells with three or more series of discs and complex medullary rays, become characteristic. _Calamites undulatum_ is abundant and seems to replace _C. Suckovii_, though _C. cannæformis_ and _C. cistii_ continue. _Sigillariæ_ become very rare, and the species of Lepidodendron are few, and mostly those with large leaf-bases. _Lepidophloios_ still continues, and _Cordaites_ abounds in some beds. The ferns are greatly reduced, though a few characteristic coal-formation species occur, and the genus _Cardiopteris_ appears. Beds of coal are rare in this formation; but where they occur there is in connection with them a remarkable anticipation of the rich coal-formation flora, which would thus seem to have existed locally in the Millstone Grit period, but to have found itself limited by generally unfavorable conditions. In America, as in Europe, it is in the north that this earlier development of the coal-flora occurs, while in the south there is a lingering of old forms in the newer beds. In Newfoundland and Cape Breton, for instance, as well as in Scotland, productive coal-beds and a greater variety of species of plants occur in this formation. [FN] "Report on Fossil Plants of the Lower Carboniferous and Millstone Grit of Canada," 1873. The following would appear to be the equivalents of this formation, in flora and geological position: 1. The Seral Conglomerate of Rogers in Pennsylvania, &c. 2. The Lower Coal-formation Conglomerate and Chester groups of Illinois (Worthen). 3. The Lower Carboniferous Sandstone of Kentucky, Alabama, and Virginia. 4. The Millstone Grit and Yoredale rocks of northern England, and the Culmiferous of Devonshire. 5. The Moor rock and Lower Coal-measures of Scotland. 6. Flagstones and Lower Shales of the south of Ireland, and Millstone Grit of the north of Ireland. 7. The Jüngste Grauwacke of the Hartz, Saxony, and Silesia. (4) _The Carboniferous Limestone Series_: This affords few fossil plants in eastern America, and in so far as known they are similar to those of the next group. In Scotland it is richer in plants, but, according to Mr. Kidston, these are largely similar to those of the underlying beds, though with some species which extend upward into the Millstone Grit. In Scotland the alga named _Spirophyton_ and _Archæocalamites radiatus_--which in America are Erian--appear in this formation. (5) _The Lower Carboniferous Sub-Flora_: