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Updated: May 25, 2025
Not only, indeed, do Sir R. Murchison's conclusions tacitly assume this doctrine of universal distribution, but he distinctly enunciates it. "The mere presence of a graptolite," he says, "will at once decide that the enclosing rock is Silurian;" and he says this, notwithstanding repeated warnings against such generalizations.
The prevailing fossils, besides corals and trilobites, and some crinoids, are several small species of Orthis, Cardiola, and numerous thin-shelled species of Orthoceratites. About six species of Graptolite, a peculiar group of sertularian fossils before alluded to as being confined to Silurian rocks, occur in this shale.
About eleven species of Graptolite are reckoned as belonging to this formation; they are chiefly found in peculiar localities where black mud abounded. The formation, when traced into South Wales and Ireland, assumes a greatly altered mineral aspect, but still retains its characteristic fossils.
COELENTERATES. This subkingdom includes two classes of interest to the geologist, the HYDROZOA, such as the fresh-water hydra and the jellyfish, and the CORALS. Both classes existed in the Cambrian. The Hydrozoa were represented not only by jellyfish but also by the GRAPTOLITE, which takes its name from a fancied resemblance of some of its forms to a quill pen.
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