which covered an interior skull, of which, save in one genus,—the _Dipterus_,—not a vestige remains in any of the Old Red fishes thus protected, that we first trace what seem to be the homologues of the cranial bones of the osseous fishes,—at least their homologues so far as the _cuticular_ can represent the _internal_. They appear for the first time, not as modified spinous processes, broadened, as in the carapace of the Chelonians, into _osseous_ plates, but like those _corneous_ external plates of this order of reptiles, (known in one species as the tortoise-shell of commerce,) the origin of which is purely cuticular, and which evince so little correspondence in their divisions with the sutures of the bones on which they rest, that they have been instanced, in their relation to the joinings beneath, as admirable illustrations of the _cross-banding_ of the mechanician. In the heads of the osseous fishes, the cranium proper, though consisting, like the skulls of birds, reptiles, and mammals, of several bones, exists from snout to nape, and from mastoid to mastoid, as one unbroken box; whereas all the other bones of the head, such as the maxillaries and intermaxillaries, the lower jaws, the opercular appendages, the branchial arches, and the branchiostegous rays, are connected but by muscle and ligament, and fall apart under the putrefactive influences, or in the process of boiling. This unbroken box, which consists, in the cod, of twenty-five bones, is the _homologue_ of that cranial box of the Placoids which consists of one entire piece, and the _homotype_, according to Oken, of the bodies and spinal processes of four vertebræ; while the looser bones which drop away represent their _ribs_. The upper surface of the box,—that extending from the nasal bone to the nape,—is the only part over which a dermal buckler could be laid, as it is the only part with which the external skin comes in contact; and so it is between this upper surface and the cranial bucklers of the earlier Ganoids that we have to institute comparisons. For it is a curious fact, that, with the exception of the Old Red genera _Acanthodus_, _Cheiracanthus_, and _Diplacanthus_,[14] all the Ganoids of the period in which Ganoids first appear _have_ dermal bucklers placed right over their true skulls, and that these, though as united in their parts as the bones proper to the cranium in quadrupeds and fishes, are composed of several pieces, furnished each with its independent centre of ossification. The Dipterians, the Cœlacanths, the Cephalaspians, and at least one genus placed rather doubtfully among the Acanths,—the genus _Cheirolepis_,—all possessed cranial bucklers extending from the nape to the snout, in which the plates, various, in the several genera, in form and position, were fast _soldered_ together, though in every instance the lines of suture were distinctly marked. [Illustration: Fig. 10. UPPER SURFACE OF CRANIUM OF COD.[15]