Strictly speaking all fish whose tails are not diphycercal have heterocercal tails, but the term is commonly applied to two-bladed tails in which the spinal column forms a definite axis running through the dorsal blade, while the ventral blade is enlarged and generally forms the functional part of the tail. Such heterocercal tails are found in nearly all Elasmobranchii, together with the living cartilaginous Ganoidei, and many extinct forms belonging to the same order; _Lepidosteus_, _Amia_, and the Dipteridae among Dipnoi, have tails which, though obviously heterocercal, are not two-bladed. The vast majority of the Teleostei and some extinct Ganoidei have heterocercal tails of the modified type to which the term =homocercal= is applied. The hypural bones which support the lower half of the tail fin become much enlarged, and frequently unite to form a wedge-shaped bone which becomes ankylosed to the last ossified vertebral centrum. The fin-rays then become arranged in such a way as to produce a secondary appearance of symmetry. Some homocercal fish such as the Perch have the end of the notochord protected by a calcified or completely ossified sheath, the =urostyle=, to which several neural and haemal arches may be attached, and which becomes united with the centrum of the last vertebra; in others such as the Salmon the end of the notochord is protected only by laterally placed bony plates. THE SKULL. It is often impossible to draw a hard and fast line between the cranium and the vertebral column. This is the case for instance in _Acipenser_ (fig. 18, 16) among Chondrostei, in _Amia_ among Holostei, and in _Ceratodus_ and _Protopterus_ among Dipnoi. The occipital region of the skull in _Amia_ is clearly formed of three cervical vertebrae whose centra have become absorbed into the cranium, while the neural arches and spines are still distinguishable. The simplest type of cranium is that found in ELASMOBRANCHS: it consists of a simple cartilaginous box, which is generally immovably fixed to the vertebral column, though in some forms, like _Scymnus_ and _Galeus_, a joint is indicated, and in others, such as the Rays, one is fairly well developed. The cranium in Elasmobranchs is never bony, though the cartilage is sometimes calcified. It is drawn out laterally into an antorbital process in front of the eye, and a postorbital process behind it. The nasal capsules are always cartilaginous, and the eye, as a general rule, has a cartilaginous sclerotic investment. The cranium is often prolonged in front into a rostrum which is enormously developed in _Pristis_ and some Rays. The cartilaginous roof of the cranium is rendered incomplete by the presence of a large hole, the anterior fontanelle. [Illustration FIG. 16. A. SKULL OF _Notidanus_ × 1/2 (Brit. Mus.). B. SKULL OF _Cestracion_ × 1/3 (after GEGENBAUR). In neither case are the branchial arches shown.