AMMONITES HUMPHRIESIANUS.
(Oolite.)
| Fig. 95. ENCRINITES MONILIFORMIS. (Trias.) | Fig. 96.
CUPRESSOCRINUS CRASSUS. (Old Red Sandstone.) |
Fig. 97.[21]
PENTACRINUS FASCICULOSUS.
(Lias.)
a, CHAMFERED SCALES. (Osteolepis.)
b, IMBRICATED SCALES. (Glyptolepis.)
(Old Red Sandstone.)
In carefully examining, for purposes of restoration, some of the earliest ganoidal fishes, I was not a little impressed by the peculiar mechanical contrivances exhibited in their largely developed dermal skeletons. In some cases these contrivances were sufficiently simple, resembling those which we find exemplified in the humbler trades, originated in comparatively unenlightened ages; and yet their simplicity had but the effect of rendering the peculiarly human cast of the mind exhibited in their production all the more obvious. The bony scales which covered fishes such as the Osteolepis and Diplopterus of the Old Red Sandstone, or the Megalichthys of the Coal Measures, were of considerable mass and thickness. They could not, compatibly with much nicety of finish, be laid over each other, like the thin horny scales of the salmon or herring; and so we find them curiously fitted together, not like slates on a modern roof, but like hewn stones on an ancient one. There ran on the upper surface of each, along the anterior side and higher end, a groove of a depth equal to half the thickness of the scale; and along the posterior side and lower end, on the under surface, a sort of bevelled chamfer, which, fitting into the grooves of the scales immediately behind and beneath it, brought their surfaces to the same line, and rendered the shining coverings of these strongly armed ganoids as smooth and even as those of the most delicately coated fishes of the present day. In the scales of the Celacanth family the arrangement was different. Though exceedingly massive in some of the genera, they were imbricated, like those of the Pangolins; and were chiefly remarkable for the combination of contrivances which they exhibited for securing the greatest possible amount of strength from the least possible amount of thickness. The scales of Holoptychius giganteus may be selected as representative of those of the family to which it belonged. It consisted of three plates, or rather, like the human skull, of two solid plates, with a diploe or spongy layer between. The outer surface was curiously fretted into alternate ridges and furrows; and hence the name of the genus,—wrinkled scale; and these imparted to the exterior plate on which they occurred, and which was formed of solid bone, the strength which results from a corrugated or fluted surface. Cromwell, in commissioning a friend to send him a helmet, shrewdly stipulated that it should be a "fluted pot;" and we find that the Holoptychius had got the principle of the fluted pot exemplified in the outer plate of each of its scales, untold ages before. The spongy middle plate must, like the diploe of the skull, have served to deaden the vibrations of a blow dealt from the outside. It was a stratum of sand bags piled up in the middle of a plank rampart. Their innermost table was formed, like the outer, of solid bone, but had a different arrangement. It was properly not one, but several tables, in each of which the osseous fibres, spread out in the general plane of the scale, lay at a diverse angle from those of the table immediately in contact with it. The principle was evidently that of the double-woven cloth, or cloth of two incorporated layers, such as moleskin, in which, from the arrangement of the threads, what a draper would term the tear of the one layer or fold lies at a different angle in the general fabric from that of the other. We are thus presented, in a single fossil scale little more than the eighth part of an inch in thickness, with three distinct strengthening principles,—the principle of Cromwell's "fluted pot,"—the principle of a rampart lined with plank, and filled with sand bags in the centre,—and the principle of the double-woven fabrics of the "moleskin" manufacturer.[22] The contrivances exemplified in the cuirass of the Pterichthys were scarce less remarkable. It was formed of bony plates, strongly arched above, but comparatively flat beneath; and along both its anterior and posterior rims a sudden thickening of the plates formed a massive band, which served to strengthen the entire structure, as transverse ribs of stone are found strengthening Gothic vaults of the Norman age. The scale covered tail of the creature issued from within the posterior rim, which formed around it a complete though irregular ring, arched above and depressed beneath; whereas the anterior rim, to which the head was attached, was incomplete when separated from it. It was, in its detached state, an arch wanting the keystone. A keystone, however, projected outwards from the occipital plate of the head; and, as it had to form at once the bond of connection between the cerebral armature of the creature and its cuirass, and to complete the arch formed by the strengthening belt or rib of the latter, it curiously combined the principle of both the dovetail of the carpenter and the keystone of the mason. Viewed from above, it was a dovetail, forming a strong attachment of the head to the body; viewed in the transverse section, it was an efficient keystone, that gave solidity and strength to the arched belt or rib. Both keystone and dovetail are comparatively simple contrivances; but I know not that they have been united in the same piece, save in the very ancient instance furnished by the strong bony plate which connected the helmet of the Pterichthys with its cuirass.