let-snails
(_Ianthina_) of the open Atlantic. The species of _Platyostoma_
(fig. 72, h) also belong to the same family; and the entire
group is continued throughout the Devonian into the Carboniferous.
Amongst other well-known Upper Silurian Gasteropods are species
of the genera _Holopea_ (fig. 72, g), _Holopella_ (fig. 72.
e), _Platyschisma_ (fig. 72, d), _Cyclonema, Pleurotomaria,
Murchisonia, Trochonema_, &c. The oceanic Univalves (_Heteropods_)
are represented mainly by species of _Bellerophon_; and the Winged
Snails, or _Pteropods_, can still boast of the gigantic _Thecoe_
and _Conularioe_, which characterise yet older deposits. The
commonest genus of _Pteropoda_, however, is _Tentaculites_ (fig.
73), which clearly belongs here, though it has commonly been
regarded as the tube of an Annelide. The shell in this group
is a conical tube, usually adorned with prominent transverse
rings, and often with finer transverse or longitudinal striae as
well; and many beds of the Upper Silurian exhibit myriads of
such tubes scattered promiscuously over their surfaces.

The last and highest group of the _Mollusca_--that of the
_Cephalopoda_--is still represented only by _Tetrabranchiate_
forms; but the abundance and variety of these is almost beyond
belief. Many hundreds of different species are known, chiefly
belonging to the straight _Orthoceratites_, but the slightly-curved
_Cyrtoceras_ is only little less common. 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 A