Page:An Introduction to the Study of Fishes.djvu/80

56 median line; it constitutes also the floor of the orbit. The pterygoid (24) (or os transversum) starts from the quadrate, and is joined by suture to the palatine, which is toothed, and reaches to the vomer and anterior frontal.

In the occipital region there are distinguished the basioccipital (5), readily recognised by the conical excavation corresponding and similar to that of the atlas, with which it is articulated through the intervention of a capsule filled with a gelatinous substance (the remains of the notochord); the exoccipitals (10), articulated, one on each side, to the basioccipital, and expanding on the upper surface of that bone, so as to meet and support the spinal column; a superficial thin lamella (13), suturally connected with the exoccipitals, not constant in fishes, and erroneously believed by Cuvier to be the petrosal (os petrosum) of higher animals; further, the paroccipitals (9), which are wedged in between the exoccipitals and supraoccipital. This last bone (8) forms the key of the arch over the occipital foramen, and raises a strong high crest from the whole length of its mesial line; a transverse supraoccipital ridge, coming from each side of the base of this spine runs outwards laterally to the external angles of the bone. The supraoccipital separates the parietals, and forms a suture with the frontals.

In front of the basioccipital the base of the skull is formed by the basisphenoid (parasphenoid of Huxley) (6). This very long and narrow bone extends from the basioccipital beyond the brain-capsule to between the orbits, where it forms the support of the fibro-membranous interorbital septum. Anteriorly it is connate with another long hammer-shaped bone (16), the vomer, the head of which marks the anterior end of the palate, and is beset with teeth. The alisphenoids (11) are short broad bones, rising from the basisphenoid; their posterior margins are suturally connected with the anterior of the basi- and ex-occipitals.