The Teleostomi are doubtless derived from sharks, their relationship being perhaps nearest to the Ichthyotomi or to the primitive chimaeras. The lowest Teleostomi retain the shark-like con dition of the upper jaw, made of palatal ele ments which, as in the Chimera, may be fused with the cranium. In the lower forms also the primitive diphycercal or protocercal form of tail is retained. as also the archipterygium or jointed axis of the paired fins, fringed with rays on one or both sides.
We may divide the teleostomes or true fishes into three sub-classes, the Crossopterygii or fringe-fins, the Dipneusti or lungfishes, and the Actsnopterygii or ray-fins. Of these, recent writers are disposed to consider the Crosso ptery gii as most primitive, and to derive from this, by separate lines, each of the remaining sub classes, as well as the higher vertebrates.
The earliest teleostomes constitute the class called after Huxley, Crossojiterygii ( Kpcleo6c, fringe;rrep6v, fin). Its essential character is the retention of the jointed pectoral fin or archipterygium, its axis fringed by series of soft rays. This char acter it shares with the Ichthyotomi among sharks, and with the Dipneusti. From the lat ter it differs in the hyostylic cranium, the lower jaw being suspended from the hyomandibular — and by the presence of distinct premaxillary and maxillary elements in the upper jaw. In these characters it agrees with the ordinary fishes. The skeleton is more or less perfectly ossified. Outside the cartilaginous skull is a bony coat of mail. The skin is covered with firm scales or bony plates. The tail is diphy cereal, straight, and ending in a point. The shoulder-girdle, attached to the cranium, is car tilaginous, but overlaid with long, bony plates, and the branchiostegals are represented by a pair of gular plates.
In the single family (Pdypteridee) repre sented among living fishes the heart has a mus cular arterial bulb with many series of valves on its inner edge, and the large air-bladder is divided into two lobes, having the functions of a lung, though not cellular as in the lung-fishes.
The fossil types are very closely allied to the lung-fishes, and the two groups have no doubt a common origin in Silurian times. It is now usually considered that the crossopterygian is more primitive than the lung-fish, though at the same time more nearly related to the ga noids, and through them to the ordinary fishes.
From the primitive Crossopterygii the step to the ancestral amphibia, which are likewise mailed and semi-aquatic, seems a very short one. It is true that most writers until recently have regarded such dipneustans as the Ceratodonti de as representing the parents of the amphib ians. But the weight of recent authority, Gill, Boulenger, Dollo and others, seems to place the point of separation of the higher vertebrates with the crossopterygians.
Cope and Woodward divide the Crossoptery gii into four orders or sub-orders, Haplistia, Rhipidistia, Actinistia and Cladistia. To the last belong the existing species (Polypterus) alone. In all these the pectorals are narrow with a single basal bone, and the nostrils, as in the dipneustans, are below the snout.
In the Haplistia the notochord is persistent, and the basal bones of dorsal and anal fins are in regular series, much fewer in number than the fin-rays. The single family Tarrassiide, re garded as lowest of the crossopterygians, are small fishes of Carboniferous Age.
In the Rhipidistia the basal bones of the median fins are found in a single piece, not sep rate as in the Haplistia. Four families are recognized, Holoptychiidce (extinct), Rhizodon tide (extinct,) Osteolepida (extinct), Onycho dontide (extinct), the first of these being con sidered as the nearest approach of the crossop terygians to the dipnoans.
In the Actinistia there is a single fin-ray to each basal bone, the axonosts of each ray fused in a single piece. The notochord is persistent, causing the backbone in fossils to appear hollow, the cartilaginous material leaving no trace in the rocks. The genera and species are numer ous, ranging from the Subcarboniferous to the Upper Cretaceous, and belonging to the single family Ccelacanthider (extinct).
In the Cladistia the axis of the pectoral limb is fan-shaped, made of two diversified bones joined by cartilage. The notochord is re stricted and replaced by ossified vertebra. The axonosts of the dorsal and anal are in regular series, each bearing a fin-ray. The order con tains the single family Polypteridce, represented by numerous species in the Nile, Senegal and Kongo rivers. In this group the pectoral fin is formed differently from that of the other cros sopterygians, being broad, its base of two diverging bones with cartilage between. This structure, more specialized than in any other of the crossopterygians or dipneustans, has been regarded by Gill and others, as above stated, as the origin of the fingered hand (chiropterygium) of the frogs and higher vertebrates. The base of the diverging bones has been identified as the antecedent of the humerus, the bones them selves as radius and ulna, while the intervening, non-ossified cartilage breaks up into carpal bones, from which metacarpals and digits ulti mately diverge. This hypothesis is at least a reasonable one. The nostrils, as in true fishes, are superior. The body in these fishes is cov ered with rhombic enameled scales, as in the garpike, the head is similarly mailed, but in dis tinction from the garpike, the anterior rays of the dorsal are developed as isolated spines.