Fig. 28.
INNER SURFACE OF CRANIAL BUCKLER OF ASTEROLEPIS.
(One fifth nat. size, linear.)
The inner surface of the cranial buckler of Asterolepis, (fig. 28,)—that which rested on the cartilaginous box which formed the creature’s interior skull,—stands out in bolder relief from the stone than its outer surface, and forms a more picturesque object. Like the inner surfaces of the bucklers of Coccosteus and Pterichthys, but much more thickly than these, it was traversed by minute channelled markings, somewhat resembling those striæ which may be detected in the flatter bones of the ordinary fishes, and which seem in these to be mere interstices between the osseous fibres. And in the plates, as in the bones, they radiate from the centres of ossification, which are comparatively dense and massy, towards the thinner overlapping edges. These radiating lines are equally well marked in the cerebral bones of the human fœtus. The three converging ridges on the outer surface we find on the inner surface also,—the lateral ones a little bent in the middle, but so directly opposite those outside, that the thickening of the buckler which takes place along their line is at least as much a consequence of their inner as of their outer elevation over the general platform. A fourth bar ran transversely along the nape, and formed the cross beam on which the others rested; for the three longitudinal ridges may be properly regarded as three strong beams, which, extending from the transverse beam at the nape to the front, where they converged like the spokes of a wheel at the nave, gave to the cranial roof a degree of support of which, from its great flatness, it may have stood in need. In cranial bucklers in which the average thickness of the plates does not exceed three eighth parts of an inch, their thickness in the centre of the ridges exceeds three quarters. The head of the largest crocodile of the existing period is defended by an armature greatly less strong than that worn by the Asterolepis of the Lower Old Red Sandstone. Why this ancient Ganoid should have been so ponderously helmed we can but doubtfully guess; we only know, that when nature arms her soldiery, there are assailants to be resisted and a state of war to be maintained. The posterior central plate, the homologue apparently of the occipital bone, was curiously carved into an ornate massive leaf, like one of the larger leaves of a Corinthian capital, and terminated beneath, where the stem should have been, in a strong osseous knob, fashioned like a pike head. Two plates immediately over it, the homologues of the superior frontal bone, with the little nasal plate which, perched atop in the middle, lay between the creature’s eyes, resembled the head and breast in the female figure, at least not less closely than those of the “lady in the lobster;” the posterior frontal plates in which the outer and nether half of the eye orbits were hollowed formed a pair of sweeping wings, and thus in the centre of the buckler we are presented with the figure of an angel, robed and winged, and of which the large sculptured leaf forms the body, traced in a style in no degree more rude than we might expect to see exemplified on the lichen-encrusted shield of some ancient tombstone of that House of Avenel which bore as its arms the effigies of the Spectre Lady. Children have a peculiar knack in detecting such resemblances; and the discovery of the angel in the cranium of the Asterolepis I owe to one of mine.
Fig. 29.
PLATE OF CRANIAL BUCKLER OF ASTEROLEPIS.
It is on this inner side of the cranial buckler, where there are no such pseudo-joinings indicated as on the external surface, that the homologies of the plates of which it is composed can be best traced. It might be well, however, ere setting one’s self to the work of comparison, to examine the skulls of a few of the osseous fishes of our coast, and to mark how very considerably they differ from one another in their lines of suture and their general form. The cerebral divisions of the conger-eel, for instance, are very unlike those of the haddock or whiting; and the sutures in the head of the gurnard are dissimilarly arranged from those in the head of the perch. And after tracing the general type in the more anomalous forms, and finding, with Cuvier, that in even these the “skull consists of the same bones, though much subdivided, as the skulls of the other vertebrata,” we will be the better qualified for grappling with the not greater anomalies which occur in the cranial buckler of the Asterolepis. The occipital plate, A, a, a, (fig. 29,) occupies its ordinary place opposite the centre of the nape; the two parietals, B, B, rest beside it in their usual ichthyic position of displacement; the superior frontal we find existing, as in the young of many animals, in two pieces, C, C; the nasal plate I, placed immediately in advance of it, is flanked, as in the cod, by the anterior frontals, D, D; the posterior frontals, F, F, which, when viewed as in the print, from beneath, seem of considerable size, and describe laterally and posteriorly about one half the eye orbits, have their area on the exterior surface greatly reduced by the overriding squamose sutures of the plates to which they join; and lastly, two of these overlying plates, E, E,—which, occurring in the line of the lateral bar or beam, are of great strength and thickness, and lie for two thirds of their length along the parietals, and for the remaining third along the superior frontals,—represent the mastoid bones. Such, so far as I have been yet able to read the cranial buckler of the Asterolepis, seem to be the homologies of its component plates.
