In the _Teleosteous_ fishes the spinous column consists of completely ossified amphicœlous vertebræ; its termination is _homocercal_--that is, the caudal fin appears to be more or less symmetrical, the last vertebra occupying a central position in the base of the fin, and being coalesced with a flat osseous lamella, the _hypural_ (Fig. 23, 70), on the hind margin of which the fin-rays are fixed. The hypural is but a union of modified hæmapophyses which are directed backwards, and the actual termination of the notochord is bent upwards, and lies along the upper edge of the hypural, hidden below the last rudimentary neural elements. In some Teleosteans, as the _Salmonidæ_, the last vertebræ are conspicuously bent upwards: in fact, strictly speaking, this homocercal condition is but one of the various degrees of heterocercy, different from that of many Ganoids in this respect only, that the caudal fin itself has assumed a higher degree of symmetry. The neural and hæmal arches generally coalesce with the centrum, but there are many exceptions, inasmuch as some portion of the arches of a species, or all of them, show the original division. The vertebræ are generally united with one another by zygapophyses, and frequently similar additional articulations exist at the lower parts of the centra. Parapophyses and ribs are very general, but the latter are inserted on the centra and the base of the processes, and never on their extremities. The point of insertion of the rib, more especially on the anterior vertebræ, may be still higher--viz. at the base of the neural arch, as in _Cotylis_ and allied genera, and even on the top of the neurapophysis, as in _Batrachus_. There is a great amount of variation as regards the degree in which the primordial cranium persists; it is always more or less replaced by bone; frequently it disappears entirely, but in some fishes, like the _Salmonidæ_ or _Esocidæ_, the cartilage persists to the same or even to a greater extent than in the Ganoidei holostei. Added to the bones preformed in cartilage are a great number of _membrane-bones_. The different kinds of these membrane-bones occur with greater or less constancy throughout this sub-class; they often coalesce with, and are no more separable from, the neighbouring or underlying cartilage-bones. All these bones have been topographically enumerated in Chapter IV. Many attempts have been made to classify the bones of the Teleosteous skull, according to their supposed relation to each other, or with the view to demonstrate the unity of plan on which the skull has been built; but in all either the one or the other of the following two principles has been followed:--