56o POPULAR SCIENCE MONTHLY, 



form the three chief groups of cat-sharks (ScyliorJmius, etc.), mackerel- 

 sharks (Lamna, etc.) and iron-sharks {Carcharhias, etc.). 



In the other group the vertebrae have their calcareous matter ar- 

 ranged in rings, one or more about the notochordal center. In all these 

 the anal fin is absent, and in the process of specialization, is formed 

 the flattened body and broad fins of the ray. This group is called 

 Tectospondyli. Hasse's Cyclospondyli (sharks with one ring of cal- 

 careous matter) constitute the most primitive extreme of a group rep- 

 resenting continuous evolution. 



From Cladoselache and Clilamydoselachus through the sharks to the 

 rays we have an almost continuous series which reaches its highest 

 development in the devil rays or mantas of the tropical seas, Manta 

 and Mohula being the most specialized genera and among the very 

 largest of the fishes. However difEerent the rays and skates may appear 

 in form and habit, they are structurally similar to the sharks and have 

 sprung from the main shark stem. 



The most ancient offshoot from the shark stem, perhaps dating 

 before Silurian times and having its root in ancient Ichthyotomi, is 

 the group of Holocephali or cliimgeras, shark-like in essentials, but dif- 

 fering widely in details. Of these there are but few living forms, and 

 the fossil types are known only from dental plates and fin spines. 

 The living forms are found in the deeper seas, the world over, the 

 most primitive genus being the newly discovered RhinocJiimwra. The 

 fusion of the teeth into overlapping plates, the covering of the gills 

 by a dermal flap, the complete union of the palatoquadrate apparatus 

 or upper jaw with the skull and the development of a peculiar clasping 

 spine on the forehead of the male are characteristic of the chimseras. 

 The group is one of the most ancient, but with the chimaeras it ends, 

 for the species has nothing in common with modern fishes except what 

 both have derived from their common ancestors the sharks. 



The most important offshoot of the primitive sharks is not the 

 chamaeras, nor even the shark series itself, but the group of dipnoans or 

 lung-fishes and the long chain of their descendants. With the dipnoan 

 appears the lung or air-bladder, at first an outgrowth from the ventral 

 side of the oesophagus, as it still is in all higher animals, but later 

 turning over, among fishes, and springing from the dorsal side. At 

 first an arrangement for breathing air, a sort of accessory gill — it 

 becomes the sole organ of respiration in the higher forms, while in the 

 bony fishes its respiratory function is lost altogether. The air-bladder 

 is a degenerate gill. In the dipnoans the shoulder girdle moves for- 

 ward to the skull, and the pectoral limb, a jointed and fringed 

 archipterygium, apparently derived from ancestors of the type of 

 Pleuracanthus is its characteristic appendage. The shark-like struc- 

 ture of the mouth remains. 



