Fig. 30.
PORTION OF UNDER JAW OF ASTEROLEPIS, (OUTER SIDE.)
(One half nat. size.)
Fig. 31.
PORTION OF UNDER JAW OF ASTEROLEPIS, (INNER SIDE.)
(One half nat. size.)
There were no parts of the animal more remarkable than its jaws. The under jaws,—for the nether maxillary consisted, in this fish, as in the placoid fishes, and in the quadrupeds generally, of two pieces joined in the middle,—were, like those of the Holoptychius, boxes of bone, which enclosed central masses of cartilage. The outer and under sides were thickly covered with the characteristic star-like tubercles; and along the upper margin or lip there ran a thickly-set row of small broadly-based teeth, planted as directly on the edge of the exterior plate as iron spikes on the upper edge of a gate (fig. 30.) Mr. Parkinson expresses some wonder, in his work on fossils, that, in a fine ichthyolite in the British Museum, not only the teeth should have been preserved, but also the lips; but we now know enough of the construction of the ancient Ganoids to cease wondering. The lips were formed of as solid bone as the teeth themselves, and had as fair a chance of being preserved entire; just as the metallic rim of a cogged wheel has as fair a chance of being preserved as the metallic cogs that project from it. Immediately behind the front row,—in which the teeth present the ordinary ichthyic appearance,—there ran a thinly-set row of huge reptile teeth, based on an interior platform of bone, which formed the top of the cartilage-enclosing box composing the jaw. These were at once bent outwards and twisted laterally, somewhat like nails that have been drawn out of wood by the claw of a carpenter’s hammer, and bent awry with the wrench, (fig. 31.) They were furrowed longitudinally from point to base by minute thickly-set striæ and were furnished laterally, in most of the specimens though not in all, with two sharp cutting edges. The reptile had as yet no existence in creation; but we see its future coming symbolized in the dentition of this ancient Ganoid: it, as it were, shows us the crocodile lying entrenched behind the fish. The interior structure of these reptile teeth is very remarkable. In the longitudinal section we find numerous cancelli, ranged lengthwise along the outer edges, but much crossed, net-like, within,—greatly more open towards the base than at the point,—and giving place in the centre to a hollow space, occasionally traversed by a few slim osseous partitions. In the transverse section these cancelli are found to radiate from the open centre towards the circumference, like the spokes of a wheel from the nave; and each spoke seems as if, like Aaron’s rod, it had become instinct with vegetative life, and had sprouted into branch and blossom. Seen in a microscope of limited field, that takes in, as in the accompanying print, (fig. 32,) not more than a fourth part of the section, the appearance presented is that of a well-trained wall tree. And hence the generic name Dendrodus, given by Professor Owen to teeth found detached in the deposits of Moray, when the creatures to which they had belonged were still unknown,—a name, however, which will, I suspect, be found synonymous rather with that of a family than of a genus; for so far as I have yet examined, I find that the dendrodic or tree-like tooth, was in at least the Old Red Sandstone, a characteristic of all the Cœlacanth family. I may mention, however, as a curious subject of inquiry, that the Cœlacanths of the Coal Measures seem to have had their reptile teeth formed of pure ivory,—a substance, which I have not yet detected among the reptile-fish of the Old Red. Towards the base of the reptile teeth of Asterolepis, the interstices between the branches greatly widen, as in the branches of a tree in winter divested of its foliage, (fig. 33, c;) the texture also opens towards the base in the fish-teeth, outside, in which, however, the pattern in the transverse section is greatly less complex and ornate than that which the reptile teeth exhibits. When cut across near the point, they appear each as a thick ring, (b,) traversed by lines that radiate towards the centre; when cut across about half way down, they somewhat resemble, seen under a high magnifying power those cast-iron wheels on which the engineer mounts his railway carriages, (a.) In the longitudinal section their line of junction with the jaw is marked by numerous openings, but by no line of division, and they appear as thickly dotted by what were once canaliculi, or life points, as any portion of the dermal bone on which they rest.
Fig. 32.