There are also numerous
forms of the genera _Phragmoceras, Ascoceras, Gyroteras, Lituites_,
and _Nautilus_. Here, also, are the first-known species of the
genus _Goniatites_--a group which attains considerable importance
in later deposits, and which is to be regarded as the precursor
of the _Ammonites_ of the Secondary period.
[Illustration: Fig. 74.--Head-shield of _Pteraspis Banksii_, Ludlow
rocks. (After Murchison.)]
[Illustration: Fig. 75.--A, Spine of _Onchus tenuistriatus_;
B, Shagreen-scales of _Thelodus_. Both from the "bone-bed" of
the Upper Ludlow rocks. (After Murchison.)]
Finally, we find ourselves for the first time called upon to
consider the remains of undoubted vertebrate animals, in the
form of _Fishes_. The oldest of these remains, so far as yet
known, are found in the Lower Ludlow rocks, and they consist of
the bony head-shields or bucklers of certain singular armoured
fishes belonging to the group of the _Ganoids_, represented at
the present day by the Sturgeons, the Gar-pikes of North America,
and a few other less familiar forms. The principal Upper Silurian
genus of these is _Pteraspis_, and the annexed illustration (fig.
74) will give some idea of the extraordinary form of the shield
covering the head in these ancient fishes. The remarkable stratum
near the top of the Ludlow formation known as the "bone-bed" has
also yielded the remains of shark-like fishes.
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