Fig. 10.
UPPER SURFACE OF CRANIUM OF COD.[15]
A, Occipital bone. B, B, Parietals. C, C, C, Superior frontal. D, D, Anterior frontal. I, Nasal bone. F, F, Posterior frontals. E, E, Mastoid bones. 2, 2, Eye orbits. a, a, Par-occipital bones.
On each side of this external cranium the various cerebral plates, like the corresponding cerebral ribs in the osseous fishes, were free, at least not anchylosed together; and some of their number unequivocally performed, in part at least, the functions of two of these cerebral ribs, viz. the upper and under jaws, with the functions of the opercular appendages attached to the latter. In the cod, as in most other osseous fishes, the upper portion of the cranium consists of thirteen bones, which represent, however, only seven bones in the human skull,—the nasal, the frontal, the two parietal, the occipital, and one-half the two temporal bones. And whereas in man, and in most of the mammals, there are four of these placed in the medial line,—the four which, according to the assertors of the vertebral theory, form the spinal crests of the four cerebral vertebræ,—in the cod there are but three. The super-occipital bone, A, (fig. 10,) pieces on to the superior frontal, C, C, C; and the parietals, B, B, which in the human subject from the upper and middle portions of the cranial vault, are thrust out laterally and posteriorly, and take their places, in a subordinate capacity, on each side of the super-occipital. This is not an invariable arrangement among fishes;—in the carp genus, for instance, the parietals assume their proper medial place between the occipital and frontal bones; but so very general is the displacement, that Professor Owen regards it as characteristic of the great ichthyic class, and as the first example in the vertebrata, reckoning from the lower forms upwards, of a sort of natural dislocation among the bones,—“a modification,” he remarks, “which, sometimes accompanied by great change of place, has tended most to obscure the essential nature of parts, and their true relations to the archetype.”
Fig. 11.
CRANIAL BUCKLER OF COCCOSTEUS DECIPIENS.
a, a, Points of attachment to the cuirass which covered the upper part of the creature’s body.
Of all the cerebral bucklers of the first ganoid period, that which best bears comparison with the cranial front of the cod is the buckler of the Coccosteus, (fig. 11.) The general proportions of this portion of the ancient Cephalaspian head differ very considerably from those of the corresponding part in the modern cycloid one; but in their larger divisions, the modern and the ancient answer bone to bone. Three osseous plates in the Coccosteus, A, C, I, the homologues, apparently, of the occipital, frontal, and nasal bones, range along the medial line. The apparent homologues of the parietals, B, B, occupy the same position of lateral displacement as the parietals of the cod and of so many other fishes. The posterior frontals, F, and the anterior frontals, D, also occupy places relatively the same, though the latter, which are of greater proportional size, encroach much further, laterally and posteriorly, on the superior frontal C, C, C, and sweep entirely round the upper half of the eye orbits, 2, 2. The apparent homologue of the mastoid bone, E, which also occupies its proper place, joins posteriorly to a little plate, a, imperfectly separated in most specimens from the parietal, but which seems to represent the par-occipital bone; and it is a curious circumstance, that as, in many of the osseous fishes, it is to these bones that the forks of the scapular arch are attached, they unite in the Coccosteus in furnishing, in like manner, a point of attachment to the cuirass which covered the upper part of the creature’s body. Of the true internal skull of the Coccosteus there remains not a vestige Like that of the sturgeon, it must have been a perishable cartilaginous box.