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. [Illustration: Fig. 99. SCALE OF HOLOPTYCHIUS GIGANTEUS. (Nat. size.) (_Old Red Sandstone._)] [Illustration: Fig. 100. SECTION OF SCALE OF HOLOPTYCHIUS. (Mag. eight diameters.)]