Ludlow rocks, remains of six genera of fish have been for a longer
period known; they belong to the order of cartilaginous fishes, an
order of mean organization and ferocious habits, of which the shark
and sturgeon are living specimens. "Some were furnished with long
palates, and squat, firmly-based teeth, well adapted for crushing the
strong-cased zoophytes and shells of the period, fragments of which
occur in the foecal remains; some with teeth that, like the fossil
sharks of the later formations, resemble lines of miniature pyramids,
larger and smaller alternating; some with teeth sharp, thin, and so
deeply serrated, that every individual tooth resembles a row of
poniards set up against the walls of an armory; and these last, says
Agassiz, furnished with weapons so murderous, must have been the
pirates of the period. Some had their fins guarded with long spines,
hooked like the beak of an eagle; some with spines of straighter and
more slender form, and ribbed and furrowed longitudinally like
columns; some were shielded by an armour of bony points, and some
thickly covered with glistening scales." {64}

The traces of fuci in this system are all but sufficient to allow of
a distinction of genera. In some parts of North America, extensive
though thin beds of them have been found. A distinguished French
geologist, M. Brogniart, has shewn that all existing marine plants
are classifiable with regard to the zones of climate; some being
fitted for the torrid zone, some for the temperate, some for the
frigid. And he establishes that the fuci of these early rocks speak
of a torrid climate, although they may be found in what are now
temperate regions; he also states that those of the higher rocks
betoken, as we ascend, a gradually diminishing temperature.

We thus early begin to find proofs of the general uniformity of