were exquisitely adapted to strengthen, by the tortuous windings of their outer edges, a shell which had to combine great lightness with great powers of resistance. Itself a continuous arch throughout, it was supported by a series of continuous arches inside, somewhat resembling in form the groined ribs of the Gothic roof, but which, unlike the ponderous stone work of the medieval architects, were as light as they were strong. And to this combination of arches there was added, in the ribs and grooves of the shell, yet another element of strength, that which has of late been introduced into iron roofs, which, by means of their corrugations, ribs and grooves like those of the ammonite, are made to span over wide spaces, without the support of beams or rafters. Still more recently, the same principle has been introduced into metallic boats, which, when corru

Fig. 94.

gated, like the old ammonites, are found to be sufficiently strong to resist almost any degree of pressure without the wonted addition of an interior framework. Similar evidences of design appear in the other extinct molluscs pecu

liar to these geologic ages, such as the hamite and turrilite. The belemnite seems to have united the principle of the float to that of the sinker, as we see both united in some of our modern life boats, which are steadied on their keel by one principle, and preserved from foundering by the other; or as we find them united by the boy in his mimic smack, which he hollows out and decks, in order to render it sufficiently light, while at the same time he furnishes it with a keel of lead, in order to render it sufficiently steady. The old articulata abound in marks of ingenious mechanical contrivance. The trilobites were covered over back and

head with the most exquisitely constructed plate armor: but as their abdomens seem to have been soft and defenceless, they had the ability of coiling themselves round on the approach of danger, plate moving on plate with the nicest

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.

a

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 cach 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.