INVERTEBRATA. 449 



This order contains a curious fish called Chimaera living 

 in European seas, and a few allied forms in other oceans. 



Order 3, Dipnoi. Fishes with persistent notochord, 

 without centra, the vertebrae represented by neural arches, 

 bearing ribs, and haemal arches. Skull cartilaginous, with 

 little ossification; paired fins pinnate in structure and 

 elongated, dermal fin-rays horny. Dermal skeleton of flat, 

 overlapping scales on the body and membrane bones over 

 the skull. Skull autostylic, palato-quadrate fixed to it. 

 Gills covered by an overculum. Single or paired lungs 

 opening ventrally into the pharynx, supplied by pulmonary 

 arteries from the fourth efferent branchial arteries, a 

 pulmonary vein opening into a separate left auricle. 



There are only three genera of Dipnoi, all living in fresh 

 water. One, Ceratodus, lives in Australia ; another, Lepi- 

 dosiren, in South America ; and the third, Protopterus, in 

 West Africa. They form an approximation in many 

 respects to the Amphibia. 



Order 4, Teleostomi. Fishes with an air-bladder homolo- 

 gous with lungs, but with veins returning blood into the car- 

 dinal veins, not into the auricle, which remains undivided. 

 Skull hyosty lie, that is jaws suspended by thehyomandibular 

 bone. A well- developed operculum, the gill-septa reduced 

 to narrow bars. Dermal skeleton of scales on the body 

 and membrane bones on the head ; along the edges of the 

 mouth membrane bones (the maxillae, premaxillae, and 

 dentaries) form the jaws and bear teeth. Paired fins 

 fan-shaped with endoskeletal rays well developed in the 

 more primitive forms, much reduced in the others. This 

 order includes both the more primitive forms formerly 

 distinguished as Ganoids and the numerous specialised 

 modern fishes called Teleosteans ; examples of the former 

 are the Sturgeons and the Bony Pike (Lepidosteus) of 

 North America; to the Teleostei belong all the more 

 common and familiar fishes such as carp, perch, cod, herring, 

 salmon, and flat-fishes. 



CLASS : AMPHIBIA. 



The frog is completely distinguished from fishes by the 

 absence of gill-slits in the adult state, but as there are 

 ZOOL. 29 



