[Footnote 15: According to Pander, the "Conodonts" are found not
only in the Lower Silurian beds, but also in the "Ungulite Grit"
(Upper Cambrian), as well as in the Devonian and Carboniferous
deposits of Russia. Should the Conodonts prove to be truly the
remains of fishes, we should thus have to transfer the first
appearance of vertebrates to, at any rate, as early a period as
the Upper Cambrian.]
CHAPTER X.
THE UPPER SILURIAN PERIOD.
Having now treated of the Lower Silurian period at considerable
length, it will not be necessary to discuss the succeeding group
of the _Upper Silurian_ in the same detail--the more so, as with a
general change of _species_ the Upper Silurian animals belong for
the most part to the same great types as those which distinguish
the Lower Silurian. As compared, also, as regards the total bulk of
strata concerned, the thickness of the Upper Silurian is generally
very much below that of the Lower Silurian, indicating that they
represent a proportionately shorter period of time. In considering
the general succession of the Upper Silurian beds, we shall,
as before, select Wales and America as being two regions where
these deposits are typically developed.
In Wales and its borders the general succession of the Upper
Silurian rocks may be taken to be as follows, in ascending order
(fig.
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