The central or Cornstone division of the system in England is characterized throughout its vast depth by a peculiar family of ichthyolites, which occur in none of the other divisions. I have already had occasion to refer to the _Cephalaspis_. Four species of this fish have been discovered in the Cornstones of Hereford, Salop, Worcester, Monmouth, and Brecon;[AL] "and as they are always found," says Mr. Murchison, "in the same division of the Old Red System, they have become valuable auxiliaries in enabling the geologist to identify its subdivisions through England and Wales, and also to institute direct comparisons between the different strata of the Old Red Sandstone of England and Scotland." The _Cephalaspis_ is one of the most curious ichthyolites of the system. (See Plate X., fig. 1.) Has the reader ever seen a saddler's cutting knife?--a tool with a crescent-shaped blade, and the handle fixed transversely in the centre of its concave side. In general outline the _Cephalaspis_ resembled this tool--the crescent-shaped blade representing the head, the transverse handle the body. We have but to give the handle an angular, instead of a rounded shape, and to press together the pointed horns of the crescent, till they incline towards each other, and the convex, or sharpened edge, is elongated into a semi-ellipse, cut in the line of its shortest diameter, in order to produce the complete form of the _Cephalaspis_. The head, compared with the body, was of great size--comprising fully one third the creature's entire length. In the centre, and placed closely together, as in many of the flat fish, were the eyes. Some of the specimens show two dorsals, and an anal and caudal fin. The thin and angular body presents a jointed appearance, somewhat like that of a lobster or trilobite. Like the bodies of most of the ichthyolites of the system, it was covered with variously formed scales of bone; the creature's head was cased in strong plates of the same material, the whole upper side lying under one huge buckler--and hence the name _Cephalaspis_, or buckler-head. In proportion to its strength and size, it seems to have been amply furnished with weapons of defence. Such was the strength and massiveness of its covering, that its remains are found comparatively entire in arenaceous rocks impregnated with iron, in which few other fossils could have survived. Its various species, as they occur in the Welsh and English Cornstones, says Mr. Murchison, seem "not to have been suddenly killed and entombed, but to have been long exposed to submarine agencies, such as the attacks of animals, currents, concretionary action," &c.; and yet, "though much dismembered, the geologist has little difficulty in recognizing even the smallest portions of them." Nor does it seem to have been quite unfurnished with offensive weapons. The sword-fish, with its strong and pointed spear, has been known to perforate the oaken ribs of the