Cephalaspis

Cephalaspis, a genus of fossil Ganoid fishes, of which six species have been described, two belonging to the Upper Silurian, and four to the Devonian measures. The head was protected by a large ganoid plate, sculptured externally with circular radiating markings. The shield was produced into a horn at each posterior corner, and bore a median and posterior dorsal spine. Agassiz gave the name cephalaspis ('buckler-headed') from this extraordinary covering, which has very much the appearance of, and was formerly supposed to be, the cephalic shield of an Asaphus or Trilobite. The body was covered with rhomboidal enamelled scales, and furnished with dorsal and pectoral fins: it terminated in a large unsymmetrical tail. In a graphic description of this fossil in his Old Red Sandstone, Miller thus sketches the general appearance of the animal: '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 resembles this tool; the crescent-shaped blade representing the head, the transverse handle the body.' The endo-skeleton was mainly cartilaginous, retaining the notochord through life. The flexible body, assisted by the large tail and the fins, would give the cephalaspis the power of moving rapidly through the water. Being a predaceous fish, it must have been a formidable enemy to its associates in the Palæozoic seas, for, besides its power of rapid motion, the sharp margin of its shield probably did the work of a vigorously hurled javelin, as in the sword-fish. Pteraspis, Asterolepis (20 to 30 feet in length), Scaphaspis, Auchenaspis, and a number of other genera, are united in the same family as Cephalaspis. See Ray Lankester, A Monograph of the Fishes of the Old Red Sand- stone in Britain, part i. 'Cephalaspidæ' (Lond. 1868-70); R. H. Traquair, The Ganoids of the British Carboniferous System (Palæont. Society, Lond. 1877).