er, lead, mercury, zinc, cobalt and other
metals, may be referable to the Devonian system.

In the centre of Europe, therefore, the Devonian rocks consist of a
vast thickness of dark-grey sandy and shaly rocks, with occasional
seams of limestone, and in particular with one thick central
calcareous zone. These rocks are characterized in the lower zones by
numerous broad-winged spirifers and by peculiar trilobites (_Phacops_,
_Homalonotus_, &c.) which, though generically like those of the
Silurian system, are specifically distinct. The central calcareous
zone abounds in corals and crinoids as well as in numerous
brachiopods. In the highest bands a profusion of coiled cephalopods
(_Clymenia_) occurs in some of the limestones, while the shales are
crowded with a small but characteristic ostracod crustacean
(_Cypridina_). Here and there traces of fishes have been found, more
especially in the Eifel, but seldom in such a state of preservation as
to warrant their being assigned to any definite place in the
zoological scale. Subsequently, however, E. Beyrich has described from
Gerolstein in the Eifel an undoubted species of _Pterichthys_, which,
as it cannot be certainly identified with any known form, he names _P.
Rhenanus_. A _Coccosteus_ has been described by F. A. Roemer from the
Harz, and still later one has been cited from Bicken near Herborn by
V. Koenen; but, as Beyrich points out, there may be some doubt as to
whether the latter is not a _Pterichthys_. A _Ctenacanthus_, seemingly
undistinguishable from the _C. Bohemicus_ of Barrande's Etage G, has
also been obtained from the Lower Devonian "Nereitenschichten" of
Thuringia. The characteristic _Holoptychius nobilissimus_ has been
detected in the Psammite de Condroz, which in Belgium forms a
characteristic sandy portion of the Upper Devonian rocks. These are
interesting facts, as helping to link the Devonian and Old Red
Sandstone types together. But they are as yet too few and unsupported
to warrant any large deduction as to the correlations between these
types.

It is in the north-east of Europe that the Devonian and Old Red
Sandstone appear to be united into one system, where the limestones
and marine organisms of the one are interstratified with the
fish-bearing sandstones and shales of the other. In Russia, as was
shown in the great work _Russia and the Ural Mountains_ by Murchison,
De Verneui