ORDER II1  .—GANOIDEI Paired fins not lobate . Mandibular arch suspended from the upper segment of the hyoid arch (hyostylic skull) . Splenial bone present . No supraoccipitai bone . Unpaired fins often with fulcra . Heart with a contractile, multivalvular conus arteriosus; intestine with a spiral valve; air-bladder with pneumatic duct communi eating with the dorsal side of the oesophagus . SUB-ORDER I.—CHONDROSTEI Pectoral arch with both clavicle and cleithrum . Ventral fir; inserted far back, with well-developed endo-skeletal rays (base, osts) ; 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 (Devonian to Jura), the mouth is toothed, praemaxillary bones are present, and the maxillaries are large, the bones of the upper surface of the head are paired, branchiostegal rays are present, and the body is covered with rhomboidal, typically ganoid bony scales .

In the

fourth family (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 sturgeons, and form a sort of connecting link between them and the Palaeoniscidae . 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; branchiostegal rays were present; scales were absent, except on the caudal lobe . In the modern Polyodontidae and Acipenseridae, whose first representatives appear in the Eocene, praemaxillaries are absent and the mouth is edentulous (Acipenseridae) or beset with minute teeth (Polyodontidae), the membrane bones of the skull are more irregular and comprise azygous elements, branchiostegal rays are absent, and the body is naked or covered with small ossifications and longitudinal series of bony scutes, whilst the caudal fin is scaled exactly as in the Palaeoniscidae . Barbels are absent in the Polyodontidae . In the Polyodontidae, represented by one species, the paddle-fish or spoon-bill (Polyodon folium), in the Mississippi, Ohio and Missouri rivers of North America, and by another (Psephurus gladius) in the Yang-tse-kiang and Hoang Ho rivers of China, the snout is produced into a very long, spatulate (Polydon) or sub-conical (Psephurus) appendage, apparently useful in stirring up the mud of the thick waters in which these fishes live, and perhaps a tactile organ compensating the very reduced size of the eyes . Psephurus gladius is said to grow to a length of 20 ft . The sturgeons (Acipenseridae) are divided into two genera: Acipenser, distributed over the coasts and fresh waters of the temperate parts of the northern hemisphere, and Scaphirhynchus, inhabiting North America and Central Asia . About twenty species of Acipenser and five of Scaphirhynchus are known . The sturgeons are of great value for their flesh, their eggs (caviare) and the isinglass from the air-bladder; several species are migratory, ascending rivers to spawn . The largest species attain a length of to to 18 ft . SUB-ORDER II.—HOLOSTEI Clavicle proper absent .

Ventral fins inserted more or less far back, without or with

mere rudiments of endo-skeletal rays; dermal rays of the dorsal and anal fins corresponding to their endoskeletal supports . Caudal fin of an abbreviate-heterocercal or homocercal type . Families: Semionotidae, Macrosemiidae, Pycnodontidae, Eugnathidae, Pachycormidae, Lepidosteidae, Aspidorhynchidae, Amiidae . First appear in the Permian with the Semionotidae, become abundant in the Trias, dominant in the Jurassic, begin to decline in the Cretaceous, and from the Eocene to the present day are reduced to the two families Lepidosteidae and Amiidae, the modern representatives of which inhabit the fresh waters of North America . In most of the Holostei the scales are bony, rhombic and covered with an enamel-like (ganoine) coating, but there is every gradation between this so-called ganoid type of scaling and the cycloid type exemplified by the Amiidae . Fulcra also disappear in some of the more specialized types . The mouth is always large and toothed, and branchiostegal rays are invariably present; a single gular plate is often present . In the earlier groups the notochord was persistent, with or without annular centra, or with each centrum composed of two elements—pleurocentrum and hypocentrum; these elements remain distinct and alternate in the caudal region of the Amiidae, whilst in the Lepidosteidae the centra are as fully developed as in most Teleosteans, and opisthocoelous or convexo-concave . The pike-like genus Lepidosteus was abundant in Europe in Eocene and Miocene times, and is now represented by three species in eastern North America, Mexico and Cuba . The largest species reaches a length of to ft . Amia, the bowfin, of similar geological age, is a much smaller fish, not exceeding 2 ft., from the eastern parts of North America . Its air-bladder is cellular and acts as an accessory breathing organ .

It deposits its eggs in a sort of

nest, which is protected by the male, who for some time accompanies the swarm of young fry and defends them with great courage . Leedsia problematica, one of the Pachycormidae from the Oxford clay, probably reached a length of 3o ft., and is the largest known Teleostome .