Skeleton well ossified; a bony operculum with branchiostegal rays; investing bones of skull and shoulder-girdle well developed; caudal fin homocercal, rarely diphycercal; optic nerves decussate; spiral valve and conus absent; anus in front of genital and urinary apertures which may be separate or form a urogenital cloaca.
This order may be subdivided into
Duct present to air-bladder; ventral fins either absent (Apodes, e. g. Muraena, Anguilla Gymnotus) or abdominal in position, e. g. Clupeidae, Sal-monidae, Cyprinidae, Siluroidei. Many are freshwater forms only.
Azygos and ventral fins with jointed fin-rays only; the ventral either jugular or thoracic; air-bladder, when present, without a duct except in Lota vulgaris, the Burbot or freshwater Cod. Gadoidei, Pleuronectoidei. Marine.
Some of the rays of the dorsal, anal, and ventral fins are spinous; scales usually ctenoid; no duct to air-bladder, e. g. Percidae, Gasterosteidae, Scomberidae, etc. Mostly marine.
Resemble 3, with which the group is often classified; but the two fifth branchial arches are fused into a single bone, e. g. Labridae or Wrasses. Marine.
Globular or much compressed laterally; skin naked or furnished with large scutes or spines; ventral fins absent or represented by spines; praemaxillae and maxillae firmly united; no duct to air-bladder, e. g. Ostracion, Diodon, Tetrodon. Marine.
Long tubular snout with terminal edentulous mouth; dermal skeleton composed of large plates; sub-opercular aperture very small; gills composed of processes arranged in tufts. Pegasidae; Syngnathidae.
2-6 are often grouped as Physoklisti.
Body covered with scales; caudal fin diphycercal; paired limbs with a central axis; with branchiae and lungs; persistent notochord without vertebral centra; a spiral valve in intestine and a cloaca; abdominal pores; a conus of large size spirally twisted. The order contains two sub-orders.
With single lung; large cycloid scales; paired limbs with bilateral rays. Ceratodus from rivers of Queensland.
Lung double; paired fins linear. Protopterus annectens and P. amphibius from tropical Africa, with five gill-clefts and a lateral membrane to the paired fins, and Lepidosiren paradoxa from Brazil, with four gill-clefts and no lateral membrane to the paired fins.
Elasmobranchii. See p. 279.
Holocephali. Leydig, Chimaera, Archiv fur Anat. und Physiol. 1851.
Ganoidei. Skull of Acipenser: structure and development, W. K. Parker, Ph. Tr. 173, 1882; Skull of Lepidosteus, Id. ibid. Structure and development of Lepidosteus, Balfour and W. N. Parker, ibid, with lit. cited. External gills of Polypterus, Steindacher, Hyrtl. SB. Akad. Wien, lx. Abth. i. 1869.
Teleostei, p. 89-90, 98, 102. Augenahnliche Organe der Fische, Leydig, Bonn. 1881; Macchie splendenti of Scopelus, Emery, Mitth. Zool. Stat. Naples, v. 1884. Leptocephalidae, Kolliker, Z. W. Z. iv. 1852; Carus, Ueber die Leptocephalidae, Leipzig, 1861. Eyes of Pleuronectidae, Agassiz, 'Young Stages,' etc, Proc. Amer. Acad. xiv. 1879.
Sexual organs of Muraenoids, Brock, Mitth. Zool. Stat. Naples, ii. 1881. Viviparous Teleostei, Ryder, Proc. U. S. Nat. Museum, viii. 1885. Relations of Yolk to Gastrula, Cunningham, Q. J. M. xxvi. Pelagic stages of Young Fish, Agassiz and Whitman, Mem. Harvard Mus. xiv. (1), 1885.
Dipnoi, Ayers, J. Z. xviii. 1885, with lit. cited. Fins of Dipnoi and species of Protopterus, Schneider, Z. A. ix. 1886. Ceratodus, Gunther, Ph. Tr. 161, 1871; Huxley, P. Z. S. 1876. Nostrils in Lepidosiren, Huxley, ibid. Skeleton and nervous system of Protopterus, Wiedersheim, J. Z. xiv. 1880; Brain, Fulliquet, Recueil Zool. Suisse, iii. 1886. Heart, branchial arteries (also in bony Ganoidei), Boas, M. J. vi. 1880. Ovarian ovum of Protopterus, Beddard, Z. A. ix. 1886.