Bony plates are developed not only on the head but also on the anterior part of the trunk, where they consist of a dorsal, a ventral, and a pair of lateral plates which articulate with the cranial shield. The posterior part of the trunk is naked. The tail is diphycercal. The jaws are shear-like, and their margins are usually provided with pointed teeth whose bases fuse with the tissue of the jaw and constitute dental plates. There seem to have been three pairs of these plates, arranged as in the Sirenoidei, the principal ones in the upper jaw being borne on the palato-pterygoids. Small pelvic fins are present, but pectoral fins are unknown. The Arthrodira occur chiefly in beds of Devonian and Carboniferous age. Two of the best known genera are _Coccosteus_ from the European Devonian and _Dinichthys_, a large predatory form from the lower Carboniferous of Ohio. FOOTNOTES: [29] See p. 79. [30] For this and other groups of extinct fish see A. Smith Woodward, _Catalogue of Fossil Fish in the British Museum_, Parts I.-III. London, 1889-95. [31] See p. 127. [32] See B. Dean, _J. Morphol._ vol. IX. pp. 87-114, 1894, and _Nat. Sci._ vol. VIII. p. 245, 1896. [33] A. Günther, _Phil. Trans._ vol. 161, Part II. 1871, p. 511. T.H. Huxley, "On Ceratodus and the classification of fishes," _P.Z.S._ 1876, p. 24. CHAPTER VI. THE SKELETON OF THE DOGFISH[34]. _Scyllium canicula._ I. EXOSKELETON. The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel. The scales are larger on the dorsal than on the ventral surface, and on the jaws they are specially large and regularly arranged in rows, there forming the teeth. The margins of the jaws or lips are without scales. A second exoskeletal structure is found in the fins, all of which, both paired and unpaired, have, in addition to their cartilaginous endoskeleton, large numbers of long slender horny fibres, the fin-rays, which are of exoskeletal origin. II. ENDOSKELETON. The endoskeleton of the dogfish consists almost entirely of cartilage, which however may become calcified in places, e.g. the centrum of each vertebra is lined by a layer of calcified tissue. The endoskeleton is divisible into an =axial= portion consisting of the vertebral column, skull, and skeleton of the median fins, and an =appendicular= portion consisting of the skeleton of the paired fins and their girdles. 1. THE AXIAL SKELETON. A. THE VERTEBRAL COLUMN AND RIBS. The vertebral column consists of a series of some hundred and thirty vertebrae, each of which is united with its predecessor and successor in such a way as to allow a large amount of flexibility.