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.)]

A brief anecdote, yet further illustrative of the framework of this
ancient ganoid, may throw some additional light on what I have ventured
to term the _human_ cast of the contrivances exhibited in the organisms
of the old geologic ages. After carefully examining many specimens, I
published a restoration of both the upper and under side of Pterichthys
fully fifteen years ago. The greatest of livi