ebrate
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.

[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. 57):--

(1) The base of the Upper Silurian series is constituted by a
series of arenaceous beds, to which the name of "May Hill Sandstone"
wa