In the HOLOCEPHALI--_Chimaera_, _Hariotta_ and _Callorhynchus_--only three pairs of teeth or dental plates occur, two pairs in the upper jaw, one in the lower. These structures persist throughout life and grow continuously. The upper tooth structures are attached respectively to the ethmoid or vomerine region of the skull, and to the palato-pterygoids. The vomerine teeth are small, while those attached to the mandible and the palato-pterygoid region are large and bear several roughened ridges adapted for grinding food. The teeth of the two opposite sides of the jaw meet in a median symphysis. The teeth of _Chimaera_ are more adapted for cutting, those of _Callorhynchus_ for crushing. Many extinct forms are known, some of whose teeth are intermediate in structure between those of _Chimaera_ and _Callorhynchus_. The teeth of GANOIDS are also extremely variable. Among living forms, the Holostei are more richly provided with teeth than are any other fishes, as they may occur on the premaxillae, maxillae, palatines, pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the Chondrostei, on the other hand, the adult Acipenseridae are toothless; small teeth however occur in the larval sturgeon, and in _Polyodon_ many small teeth are found attached merely to the mucous membrane of the jaws. Many fossil Ganoids have numerous flattened or knob-like teeth, borne on the maxillae, palatines, vomers and dentaries. Others have a distinctly heterodont dentition. Thus in _Lepidotus_ the premaxillae bear chisel-like teeth, while knob-like teeth occur on the maxillae, palatines and vomers. In _Rhizodus_ all the teeth are pointed, but while the majority are small a few very large ones are interspersed. In TELEOSTEANS, too, the teeth are eminently variable both in form and mode of arrangement. They may be simple and isolated, or compound, and may be borne on almost any of the bones bounding the mouth cavity, and also as in the Pike, on the hyoid and branchial arches. The splenial however never bears teeth and the pterygoid and parasphenoid only rarely, thus differing from the arrangement in the Holostei. The isolated teeth are generally conical in form and are ankylosed to the bone that bears them. Such teeth are, with a few exceptions such as _Balistes_, not imbedded in sockets nor replaced vertically.