Fig. 12.
CRANIAL BUCKLER OF OSTEOLEPIS.
In the Osteolepis,—an animal the whole of whose external head I have, at an expense of some labor, and from the examination of many specimens, been enabled to restore,—the cranial buckler (fig. 12) was divided in a more arbitrary style; and we find that an element of uncertainty mingles with our inferences regarding it, from the circumstance that some of its lines of division, especially in the frontal half, were not real sutures, but formed merely a kind of surface-tatooing, resorted to as if for purposes of ornament. The cranial buckler of the Asterolepis exhibited, as I shall afterwards have occasion to show, a similar peculiarity;—both had their pseudo-sutures, resembling those false joints introduced by the architect into his rusticated basements, in order to impart the necessary aspect of regularity to what is technically termed the coursing and banding of the fabric. We can however, determine, notwithstanding the induced obscurity that the buckler of the Osteolepis was divided transversely in the middle into two main parts or segments,—an occipital part, C, and a frontal part, A; and that the occipital segment seems to include also the parietal and mastoid plates, and the frontal segment to comprise, with its own proper plates, not only the nasal plate, but also the representative of the anterior part of the vomer. All, however, is obscure. But in our uncertainty regarding the homologies of the divisions of this dermal buckler, let us not forget the homology of the buckler itself, as a whole, with the upper surface of the true cranium in the osseous fishes. Though frequently crushed and broken, it exists in all the finer specimens of my collection as a symmetrically arranged collocation of enamelled plates, as firmly united into one piece, though they all indicate their distinct centres of ossification, as the corresponding surface of the cranium in the carp or cod. The lateral curves in the frontal part immediately opposite the lozenge-shaped plate in the centre, show the position of the eyes, which were placed in this genus, as in some of the carnivorous turtles, immediately over the mouth,—an arrangement common to almost all the Ganoids of the Lower Old Red Sandstone. The nearly semicircular termination of the buckler formed the creature’s snout; and in the Osteolepis, as in the Glyptolepis and the Diplopterus, it was armed on the under side, like the vomer of so many of the osseous fishes, with sharp teeth. Some of my specimens indicate the nasal openings a little in advance of the eyes. The nape of the creature was covered by three detached plates, (9, 9, 9, fig. 13,) which rested upon anterior dorsal scales, and whose homologies, in the osseous fishes, may possibly be found in those bones which, uniting the shoulder-bones to the head, complete the scapular belt or ring. The operculum we find represented by a single plate (8) which had attached to it, as its sub-operculum, a plate (13) of nearly equal size, (see figs. 14 and 15.) Four small plates (2, 4, 5) formed the under curve of the eyes, described in many of the osseous fishes by a chain of small bones or ossicles; a considerably larger plate (6) occupied the place of the preopercular bone; while the intermaxillaries had their representatives in well-marked plates, (3, 3,) which, in the genera Osteolepis, Diplopterus, and Glyptolepis, we find bristling so thickly with teeth along their lower edges, as to remind us of the miniature saws employed by the joiner in cutting out circular holes. These external intermaxillaries did not, as in the perch or cod, meet in front of the nasal bone and vomer, but joined on at the side, a little in advance of the eyes, leaving the rounded termination of the cranial buckler, which, like the intermaxillaries, was thickly fringed with teeth, to form, as has been already said, the creature’s snout.
Fig. 13.
UPPER PART OF HEAD OF OSTEOLEPIS.
The under jaws (10)—strongly-marked bones in at least all the Dipterian and Cœlacanth genera—we find represented externally by massy plates, bearing, like those of the upper jaw, their range of teeth. As shown in a well-preserved specimen of the lower jaw of Holoptychius, in my possession, they were boxes of bone enclosing a bulky nucleus of cartilage, which, in approaching towards the condyloid process, where great strength was necessary, was thickly traversed by osseous cancelli, and passed at the joint into true bone. It is in the under jaws of the earlier Ganoids that we first detect a true union of the external with the internal skeleton,—of the bony plates and teeth, which were mere plates and teeth of the skin, with the osseous, granular walls which enclosed at least all the larger pieces of the cartilaginous framework of the interior. The jaws of the Rays and Sharks, formed of cartilage, and fenced round on their sides and edges by their thin coverings of polygonal, bony points, are wholly internal and skin-covered; whereas the teeth, which rest on the soft cuticular integument right over them, are as purely dermal as the surrounding shagreen. Teeth and shagreen may, we find, be alike stripped off with the skin. Now, in the earlier ganoidal jaw, two sides of the osseous box which it composed,—its outer and under sides,—were mere dermal plates, representative of the skin of the placoids, or of their shagreen; while the other two,—its upper and inner sides,—seem to have been developments of the interior osseous walls which covered the endo-skeletal cartilage. Nor is it unworthy of notice, that the reptile fishes of the period had their ichthyic teeth ranged along the edge of an exterior dermal plate which covered the outer side of the jaw; whereas their reptile teeth were planted on a plate, apparently of interior development, which covered its upper edge. It is further worthy of remark, that while the teeth of the dermal plate,—themselves also dermal,—seem as if they had grown out of it, and formed part of it,—just as the teeth of the Placoids grew out of the skin on which they rest,—the reptile teeth within rested in shallow pits,—the first faint indications of true sockets.
