adjustment, till the rim of the armed tail rested on that of the armed head, and the creature presented the appearance of a ball defended at every point. In some genera, as in Calymene, the tail consisted of jointed segments till its termination; in others, as in Illænus, there was a great caudal shield, that in size and form corresponded to the shield which covered the head; the segments of Calymene, from the flexibility of their joints, fitted close to the cerebral rim; while the same effect was produced in the inflexible shields, caudal and cephalic, of Illanus, by their exact correspondence, and the flexibility of the connecting rings, which enabled them to fit together like two equal-sized cymbals brought into contact at every point by the hand. Nor were the ancient crinoids less remarkable for the amount of nice contrivance which their structures exhibited, than the ancient molluscs or crustaceans. In their calyx-like bodies, consisting always of many parts, we find the principle of the arch introdued in almost every possible form and modification, and the utmost flexibility secured to their stony arms by the amazing number of the pieces of which they were composed, and the nice disposition of the joints. In the Pentacrinites of the Secondary period (see Fig. 97) an immense spread of arms, about a thousand in number, and composed of about a hundred thousand separate pieces, had all the flexibility, though formed of solid lime, of a drift of nets, and yet were so nicely jointed, tooth fitting into tooth in all their numerous parts, and the whole so bound together by ligament, that, with all the flexibility, they had also all the toughness and tenacity, of pieces of thread network. Human ingenuity, with the same purposes to effect, that is, the sweeping of shoals of swimming animals into a central receptacle, would probably construct a somewhat similar machine; but it would take half a lifetime to execute one equally elaborate.

In carefully examining, for purposes of restoration, some of the earliest ganoidal fishes, I was not a little impressed

* a, Articulating surface of joint. b, Fragment of column, exhibiting laterally the tooth processes, so fitted into each other as to admit of

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

flexure without risk of dislocation. The uppermost joint shows two lateral cavities for the articulation of auxiliary arms.

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