Page:An Introduction to the Study of Fishes.djvu/115

Rh visceral skeleton of the skull. Further, a distinction is made between the bones preformed in cartilage and those originating in tegumentary or membranous tissue. It is admitted that the primordial cranium is a coalition of several segments, the number of which is determined by that of the visceral arches, these representing the hæmal arches of the vertebral column; but the membrane-bones are excluded from a consideration of the vertebral division of the primordial skull, as elements originally independent of it, although these additions have entered into special relations to the cartilage-bones.

With these views the bones of the Teleosteous skull are classified thus:—

1. Cartilage-bones of the primordial skull.—The basi-occipital (5 in Figs. 23–26) has retained the form of a vertebral centrum; it is generally concave behind, the concavity containing remains of the notochord; rarely a rounded articulary head of the first vertebra fits into it, as in Symbranchus, and still more rarely it is provided with such an articulary head (Fistularia); frequently it shows two excavations on its inner surface for the reception of the saccus vestibuli. The exoccipitals (10) are situated on the side of the basi-occipital, and contribute the greater portion of the periphery of the foramen magnum; frequently they articulate with the first vertebra, or meet in the upper median line, so as to exclude the supraoccipital from the foramen magnum. The supraoccipital (8) is intercalated between the exoccipitals, and forms a most prominent part by the median crest, which sometimes extends far forwards on the upper side of the skull, and offers attachment to the dorsal portion of the large lateral muscle of the trunk. When the interior portions of this bone remain cartilaginous, some part of the semicircular canals may be lodged in it.

The region of the skull which succeeds the bones described encloses at least the greater portion of the labyrinth, and its component parts have been named with reference to it by