512 DIPNOI [CH. 



pair of gill pouches the external openings of which have become lost. 

 In Polypterus> Protopterus and Lepidosiren both lungs are preserved 

 and, as in land animals, they join a duct which opens into the 

 pharynx by a median ventral opening. In Ceratodus the opening 

 is also median and ventral but. there is only one lung. In the other 

 Osteichthyes, it is probable that only one lung has been preserved 

 but the opening has become shifted to the mid- dorsal line. Finally 

 the circumstance that all archaic Osteichthyes use the air-bladder 

 for respiration leads to the suggestion that the group was evolved in 

 shallow waters near land and has since re-invaded the sea, gradually 

 displacing its original inhabitants the Chondrichthyes. 



CLASSIFICATION. 



Class PISCES. 



Sub-class I. CHONDRICHTHYES. Pisces in which the skeleton is 

 formed of cartilage strengthened by deposits of calcareous matter 

 but in which no true bone is developed. The skin is protected only 

 by placoid denticles composed of dentine. There is no air-bladder, 

 the nostril is undivided and the eggs are few and large, a large part 

 or the whole of development being completed before laying. 



Order I. Elasmobranchii. Chondrichthyes in which there is 

 no common gill-cover or operculum, in which the skin is covered 

 with placoid denticles and in which the upper jaw, though it may 

 articulate directly with the skull, is not fused with it. The noto- 

 chordal sheath is segmented into distinct centra. 



Sub-order 1. Selachoidei. Active free-swimming Elasmo- 

 branchii with cylindrical or spindle-shaped bodies in which the gill- 

 slits are placed above the level of insertion of the pectoral fin which 

 is of moderate size and not joined to the skull. 



Ex. Hexanchus, Heptanckus, Heterodontus (Cestracion), Scyl- 

 lium, Squalus, Galeus. 



Sub-order 2. Batoidei. Sluggish bottom-living Elasmo- 

 branchii with flattened bodies. Openings of the gill-slits situated 

 on the ventral surface beneath the insertion of the pectoral fin which 

 is of great size and the front end of which is joined to the skull, 



Ex, Raia t Trygon, Pristis. 



