In _Clymenia_, on the
other hand, whilst the shell (fig. 101) is coiled into a flat
spiral, and the partitions or septa are simple or only slightly
lobed, there is still this difference, as compared with the
_Nautilus_, that the tube of the siphuncle is placed on the _inner_
or concave side of the shell. The species of _Clymenia_ are
exclusively Devonian in their range; and some of the limestones
of this period in Germany are so richly charged with fossils of
this genus as to have received the name of "Clymenien-kalk."
The sub-kingdom of the _Vertebrates_ is still represented by
_Fishes_ only; but these are so abundant, and belong to such
varied types, that the Devonian period has been appropriately
called the "Age of Fishes." Amongst the existing fishes there are
three great groups which are of special geological importance,
as being more or less extensively represented in past time. These
groups are: (1) The _Bony Fishes_ (_Teleostei_), comprising most
existing fishes, in which the skeleton is more or less completely
converted into bone; the tail is symmetrically lobed or divided
into equal moieties; and the scales are usually thin, horny,
flexible plates, which overlap one another to a greater or less
extent. (2) The _Ganoid Fishes_ (_Ganoidei_), comprising the modern
Gar-pikes, Sturgeons, &c., in which the skeleton usually more or
less completely retains its primitive soft and cartilaginous
condition; the tail is generally markedly unsymmetrical, being
divided into two unequal lobes; and the scales (when present)
have the form of plates of bone, usually covered by a layer of
shining enamel.
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