latter serpentiform and devoid of ventrals. We now know ten
species of Palypterus, from the Nile, the Congo, the rivers of West
Africa, and lakes Chad, Rudolf and Tanganyika, and one 'of Calamichthys,
which inhabits West Africa from the Niger delta to the
Chiloango. The largest species of Polypterus reach a length of
nearly 4 ft. The young are provided with an external opercular
gill very similar to the gills of larval salamanders. The air-bladder
acts as an accessory breathing organ, although these fishes are not
known ever to leave the water. The development is stated by the
late J. S. Budgett to be even more Batrachian-like than that of the
Dipneusti, but the results of the study of the material collected by
him shortly before his death have not yet been published.
Order II.—DIPNEUSTI
Often called Dipnoi, a term proposed for this order by Müller in 1845, but which had already been used for the Batrachians (F. Leuckart, 1821) before the discovery of Lepidosiren. The substitute Dipneusti (E. Haeckel, 1866) is, therefore, preferable. Paired fins lobate, or reduced to a jointed endo-skeletal axis. Upper segment of the mandibular arch confluent with the skull (autostylic skull). Praemaxillary and maxillary bones absent, dentary absent or small and toothless; teeth on the palato-ptegoid and splenial bones, sometimes also on the vomers. No supra occipital bone. Heart trilocular, with a contractile, multivalvular conus arteriosus; intestine with a spiral valve; air-bladder transformed into a single or double lung, opening at the glottis on the ventral side of the pharynx.
The cranial roof-bones include median as well as paired pilates, which cannot easily be homologized with those of other eleostomes; in the older forms, these bones are small and numerous, and, coated with ganoine, appear on the surface of the head, whilst in the later forms they are reduced in number as well as in the degree of ossification, and have sunk below the skin. Pectoral arch with both clavicle and c lei thrum. Ventral fins inserted far back. Vertebrae acentrous. Dermal rays of vertical fins much more numerous than their supports, which correspond in number to the neural and haemal arches. Nostrils on the lower side of the snout, the posterior within the mouth. Scales cycloid, (almost quadrate in Sagenodus, a genus of Ctenodontidae). Families: Dipteridae, Ctenodontidae, Uronemidae, Ceratodontidae, Lepidosirenidae.
The Dipteridae are heterocercal and have two dorsal fins, as in the Crossopterygian Holoptychidae; in the other families the dorsal fin is elongate and single, and extends to the extremit of the tail, which belongs to the diphycercal type. In the three fiist families, which are entirely Palaeozoic, ranging from the Devonian to the Permian, the dental plates nearly always exhibit more or less clearly the points of the separate denticles of which, as shown by the development of Neoreratodus, they were originally composed, but vomerine teeth, such as exist in the Ceratodontidae and Lepidosirenidae, do not appear to have existed. In the Dipteridae alone the scales were covered with dense, punctate ganoine, which has become much reduced or disappeared entirely in the other members of this order. The two first families had well-developed gular plates.
In the Ceratodontidae, which first appeared in the Trias and have persisted to the present day, the skull is more feebly ossified than in the earlier forms, and this may well be looked upon as a degeneration, since the head of the Triassic Ceratodus sluri, whilst exhibiting the same arrangement of bones as in the living form, differs in its higher degree of ossification; and as the dermal rays of the caudal fin also exhibit distinctive features in a fossil of the same period, it is advisable to refer the existing Ceralodus forsleri to a distinct genus, which has been named Neoceratodus by Castelnau (1876) and Epiceratodus by Teller (1891). But there can be no question that Neoceratodus is very closely related to Ccratodus. lts only known species, N. forsteri, variously known as the barramunda, flat-head, and Dawson or Burnett salmon, inhabits the Burnett, Dawson and Mary rivers in Queensland, and was first discovered in 1870. lts anatomy was made known by the memoir of A. Gunther, and numerous contributions by T. H. Huxley, E. R. Lankester, J. E. V. Boas, W. B. Spencer and others, whilst its development has been elaborately worked out by R. Semon. This fish, which grows to a length of 6 ft., has the body moderately elongate and compressed, covered with large thin scales, and the paired fins are acutely lobate, consisting of a median jointed axis fringed on each side by a series of radialia supporting nne dermal rays (archipterygium of C. Gegenbaur). Although provided with a lung, which is single, Neoceratodus never leaves the water. It feeds on both animal and vegetable matter, the specimens kept in the London Zoological Gardens readily eating lettuce in addition to frogs and bits of raw meat. The early development resembles very closely that of Batrachians, but there are no metamorphoses properly speaking, and at no period does the young possess external gills ora holder or cement or an. The South American Lepidosiren and the tropical African Protoptefus, which constitute together the family Lepidosirenidae, were discovered long before Neoceralodus, the former in 1836, the latter in 1839. These fishes are much more specialized than are the Ceratodontidae; the body is more or less eel-shaped, the scales are thinner, the paired fins are reduced to slender styliform appendages formed of a jointed axis with or without a unilateral fringe of cartilaginous rays bearing fine dermal rays, and-the lung is paired. The development is even more Batrachian-like than that of Neoceratodus, and the larvae are provided with a cement organ and four (Lepidosiren) or five (Protoplerus) fringed external gills, traces of which may persist throughout life in Protopterus. The habits and development of Lemdosiren have been investigated by J. Graham Kerr, and those of Protopterus by J. S. Budgett. In oth the eggs are deposited in nests in the water and the male keeps guard over the eggs and young. The food is both animal and vegetable, as in Neoceratodus. During the dry season, Protopteru; burrows in the mud of drying marshes and, surrounded by a cocoon formed of hardened mucus secreted by glands of the skin, it s nds weeks or months in a dormant condition, breathing exclusively; by its lungs; dry clay balls containing such cocoons have often been brought over to Europe, and when soaked in water, the Protopterus is released in a most lively condition. Three species of Protopterus are known from different parts of Africa, the type species being P. annectens, an inhabitant of West Africa, from the Senegal to the Niger, and Lake Chad. Of Lepfidosiren only one species is known, L. paradoxa, living in the Amazon and Paraguay basins.
Great uncertainty, and much difference of opinion among palaeichthyologists, still revail as to the position in the system of a group of Devonian fiishes, of which Coccosteus and Dinickthys are the best-known representatives. Long placed with the Ostracophores is a group instituted by Sir F. McCoy in 1848 under the name of Placodermi; they were removed from their vicinity by A. S. Woodward in 1889, and referred to the Dipnoans as an order which he proposed to call Arthrodira. This view was based mainly on the assumption that the skull was autostylic and that maxillary bones were not develo ed, and also on the resemblance, previously noticed by J. S. Newberry, between the dentition of Dinichthys and that of Propterus. Woodward's proposal has not met with general acceptance, but it is strongly supported by the recent investigations of C. R. Eastman, who has added fresh arguments in favour of the autostylic condition of the skull and the homology of the cranial roof-plates with those of the Dipnoans, the Ceratodontidae in particular. On the other side, B. Dean and L. Hussakof deny such homologies, and even regard the dental mechanism of the Arthrodira as something quite different from the jaws and teeth of other vertebrates, and revert to the view of McCoy in lacing the Arthrodira in a group Placodermata, which they regardg as a class co-ordinate in rank with such divisions as Cyclostomi and Pisces.
In the present state of our knowledge it is perhaps best to leave the Arthrodira with or near the Dipneusti. They are thus defined by Woodward: Fishes with both head and trunk armoured, in the more specialized genera the shield of' the abdominal re ion articulating with the head-shield in ginglymoid facettes %Gr. γίγγλυμος, a hinge) which admit of free motion (hence the name Arthrodira, joint-neck). No trace of a hyomandibular bone. jaws paralleled by those of the existing Dipneusti. Notochord persistent. Pectoral fins unknown; ventral fins rudimentary. Two families: Coccosteidae and Dinichthyidae, from the Devonian of Europe and North America.
Some of the species of Dinichythys reached a great size, the head sometimes measuring a metre across.
Order III.—GANOIDEI
Paired fins not lobate. Mandibular arch suspended from the upper segment of the hyoid arch (hyostylic skull). Splenial bone present. No supra occipital bone. Unpaxred fins often with fulcra Heart with a contractile, multivalvular conus arteriosus; intestine, with a spiral valve; air-bladder with pneumatic duct communicating with the dorsal side of the oesophagus.
Sub-order I.—CHONDROSTEI
Pectoral arch with both clavicle and c lei thrum. Ventral fin; inserted far back, with well-developed endo-skeletal rays (baseosts); dermal rays of the dorsal and anal fins more numerous than their endo-skeletal supports. Heterocercal. Vertebrae acentrous.
Families: Palaeoniscidae, Platysomidae Catopteridae, Belonorhynchidae, Chondrosteidae, Polyodontidae, Acipenseridae.
In the three first families (evonian to Jura), the mouth is and the maxillaries are the head are paired, body is covered with In the fourth family toothed, praemaxillary bones are present, large, the bones of the upper surface of branchiostegal rays are present, and the rhomboidal, typically ganoid bony scales. (Trias to Lias), the snout is much elongate, and longitudinal series of scutes extend along the body, one on the back, one on the belly, and one on each side. The Liassic Chondrosteidae show an approach to the stur ons, and form a sort of connecting link between them and the Palaieoniscidae. The mouth was edentulous, praemaxillary bones were absent, but the maxillary bone was well developed, though small; the membrane bones of the skull were paired;
