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
corrugated, 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 peculiar 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 Illaenus, 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 Illaenus, 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