Some of these
belong to _Cyrtoceras_, which only differs from _Orthoceras_ in
the bow-shaped form of the shell; others belong to _Phragmoceras_,
_Lituites_, &c.; and, lastly; we have true _Nautili_, with their
spiral shells, closely resembling the existing Pearly Nautilus.
Whilst all the sub-kingdoms of the Invertebrate animals are
represented in the Lower Silurian rocks, no traces of Vertebrate
animals have ever been discovered in these ancient deposits,
unless the so-called "Conodonts" found by Pander in vast numbers
in strata of this age [15] in Russia should prove to be really
of this nature. These problematical bodies are of microscopic
size, and have the form of minute, conical, tooth-shaped spines,
with sharp edges, and hollow at the base. Their original discoverer
regarded them as the horny teeth of fishes allied to the Lampreys;
but Owen came to the conclusion that they probably belonged to
Invertebrates. The recent investigation of a vast number of similar
but slightly larger bodies, of very various forms, in the
Carboniferous rocks of Ohio, has led Professor Newberry to the
conclusion that these singular fossils really are, as Pander
thought, the teeth of Cyclostomatous fishes. The whole of this
difficult question has thus been reopened, and we may yet have
to record the first advent of Vertebrate animals in the Lower
Silurian.
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