slates, and phyllites. The central zone contains thick masses of limestone, often full of corals and shells, while the upper portions comprise thin-bedded sandstones, shales, and limestones. These various strata represent the sediments intermittently laid down upon the bottom of the sea which then covered the greater part of Europe. Here and there, they include bands of diabase and tuff, which show that submarine volcanic eruptions took place during their deposition.

In the north-west of Europe, however, the floor of the Silurian sea was irregularly ridged up into land, and large lakes were formed, into which rivers from the ancient northern continent poured enormous quantities of gravel, sand, and silt. The sites of these lakes can be traced in Scotland, the north of England, and Ireland. Similar evidence of land and lake-waters is found in New Brunswick and Nova Scotia. That some of the larger lakes were marked by lines of active volcanoes is well shown in Central Scotland, where the piles of lava and ashes left by the eruptions are more than 6000 feet thick.

The occurrence of both marine and lacustrine deposits is of the highest interest, for, on the one hand, we learn what kinds of animals lived in the sea in succession to those that peopled the Silurian waters, and, on the other hand, we meet with the first abundant remains of the vegetation that covered the land, and of the fishes that inhabited the fresh waters. The terrestrial flora of the Devonian period has been only sparingly preserved in the marine beds; but occasional drifted specimens occur to show that land was not very distant from the tracts on which these beds were laid down. In the lacustrine strata or Old Red Sandstone of Britain more abundant remains have been met with, but the chief sources of information regarding this flora are to be sought in New Brunswick and Gaspé, where upwards of

FIG. 131.-Plants of the Devonian period; (a) Psilophyton (1); (b) Palæopteris (1).

100 species of plants have been discovered. Both in Europe and in North America the Devonian vegetation was characterised by the predominance of ferns, lycopods, and calamites. It was essentially acrogenous that is, it consisted mainly of flowerless plants like our modern ferns, clubmosses, and horse-tail reeds. Traces of coniferous plants, however, show that on the uplands of the time pine-trees grew, the stems of which were now and then swept down by floods into the lakes or the sea. The general aspect of the flora must have been uniformly green and somewhat monotonous; yet we know that these early woodlands were not without insect life. Neuropterous and orthopterous wings have been preserved in the strata. Some of these indicate the existence of ancient forms of ephemera or May-fly, one of which was so large as to have a spread of wing measuring 5 inches across. There were likewise millipedes, which fed on the decayed wood of these primeval forests. Traces of land-snails too have been detected among the fossil vegetation. It is evident, however, that the plant and animal life of the land has only been sparingly preserved; and though our knowledge of it has in recent years been largely increased, we shall probably never discover more than a mere fragmentary representation of what the original terrestrial flora and fauna really were.

The lake-basins of the Old Red Sandstone have yielded large numbers of remains of the fishes of the time. They are members of the remarkable order of Ganoids-the earliest known type of fishes-which, though so abundant in early geological time, is represented at the present day by only a few widely scattered species, such as the sturgeon, the polypterus of the Nile, and the bony pike or gar-pike of the American lakes. These modern forms are denizens of fresh water, and there is reason to believe that their early ancestors were also inhabitants of lakes and rivers, though many of them may also have been able to pass out to the sea. The ganoids are so named from the enamelled scales and plates of bone in which they are encased. In some of the fossil forms, this defensive armour consisted of accurately fitting and overlapping scales (Figs. 132, 133); in others, the head

FIG. 132. Overlapping scales of with more or less of the body an Old Red Sandstone fish (Hol- was protected by large and optychius Andersoni, natural size). thick plates of bone (Fig. 134). Examples of both these kinds of armature are to be observed among the fishes of the Old Red Sandstone. Some of the

most characteristic scale-covered genera are Osteolepis, Diplopterus, Glyptolæmus, Holoptychius, Acanthodes (Figs. 132,

FIG. 133. Scale-covered Old Red Sandstone fishes; (a) Osteolepis ; (b) Acanthodes (both reduced).

133). The acanthodians (Fig. 133, b), distinguished by the thorn-like spines supporting their fins, reached their greatest

FIG. 134.-Plate-covered Old Red Sandstone fishes; (a) Cephalaspis ; (b) Pterichthys (both reduced).

development during the Devonian period. Of the platecovered ganoids or placoderms some of the most character

istic were the curious Cephalaspis (Fig. 134, a), with its headbuckler shaped like a saddler's awl, the Pteraspis, which, with Cephalaspis, had already appeared in the Silurian period, the Coccosteus and Pterichthys (Fig. 134, 6). Some of the contemporaries of these creatures attained a great size. Thus the Asterolepis had its head and shoulders encased in a buckler, which in some examples is 20 inches long by 16 broad. Still larger were some of its American allies, one of which, the Dinichthys, had a head-buckler 3 feet long armed with formidable teeth.

One of the fishes of the Old Red Sandstone, named Dipterus, has recently been found to have a singular modern representative in the barramunda or mud-fish (Ceratodus) of the Queensland rivers in Australia. It resembled the ganoids in its external enamel and strong bony helmet, but its jaws present the characteristic teeth, and its scales have the rounded or "cycloid" form of Ceratodus. That some of these fishes swarmed in the waters of the Old Red Sandstone is shown by the prodigious numbers of their remains occasionally preserved in the sandstones and flagstones. Their bodies lie piled on each other in such numbers, and often so well preserved, as to show that probably the animals were suddenly killed, and were covered up with sediment before their remains had time to decay and to be dispersed by the currents of water. Perhaps earthquake shocks, or the copious discharge of mephitic gases, or other sudden baneful influence may have been the cause of the extensive destruction of life in these ancient waters.

That some of the fishes found their way to the sea, as our modern salmon does, is indicated by the occasional occurrence of their remains among those of the truly marine fauna of the Devonian rocks. But the rarity of their