Page:An Introduction to the Study of Fishes.djvu/113

Rh 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:—

A. The "vertebral doctrine" starts from the undeniable fact that the skull is originally composed of several segments, each of which is merely the modification of a vertebra. The component parts of such a cranial segment are considered to be homologous to those of a vertebra. Three, four, or five cranial vertebræ have been distinguished, all the various bones of the fully-developed and ossified skull being referred, without distinction as to their origin, to one or the other of those vertebral segments. The idea of the typical unity of the osseous framework of Vertebrates has been worked out with the greatest originality and knowledge of detail, by Owen, who demonstrates that the fish-skull is composed of four vertebræ.

The bones of the fish-skull are, according to him, primarily divisible into those of the neuroskeleton, splanchnoskeleton, and dermoskeleton.

The bones of the neuro- or proper endo-skeleton are arranged in a series of four horizontally succeeding segments: the occipital, parietal, frontal, and nasal vertebræ; each segment consisting of an upper (neural) and a lower (hæmal) arch, with a common centre, and with diverging appendages.

The neural arches of the four vertebræ, in their succession from the occiput towards the snout, are:—

1. Epencephalic arch, composed of the occipitals.