Page:Popular Science Monthly Volume 60.djvu/568

560 form the three chief groups of cat-sharks (Scyliorhinus, etc.), mackerelsharks (Lamna, etc.) and iron-sharks (Carcharhias, etc.).

In the other group the vertebræ have their calcareous matter arranged 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 calcareous matter) constitute the most primitive extreme of a group representing continuous evolution.

From Cladoselache and Chlamydoselachus 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 Mobula being the most specialized genera and among the very largest of the fishes. However different 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 chimæras, shark-like in essentials, but differing 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 Rhinochimæra. 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 chimæras. The group is one of the most ancient, but with the chimæras 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 chamæras, 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 forward 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 structure of the mouth remains.