he wide-ranging _Coccosteus_
with _Homosteus_ and _Dinichthys_, the largest fish of the period. The
latter probably reached 20 ft. in length; it was armed with
exceedingly powerful jaws provided with turtle-like beaks. Sharks were
fairly prominent denizens of the sea; some were armed with cutting
teeth, others with crushing dental plates; and although they were on
the whole marine fishes, they were evidently able to live in fresher
waters, like some of their modern representatives, for their remains,
mostly teeth and large dermal spines, are found both in the Devonian
and Old Red rocks. _Mesacanthus_, _Diplacanthus_, _Climatius_,
_Cheiracanthus_ are characteristic genera. The crossopterygians,
ganoids with a scaly lobe in the centre of the fins, were represented
by _Holoptychius_ and _Glyptopomus_ in the Upper Old Red, and by such
genera as _Diplopterus_, _Osteolepis_, _Gyroptychius_ in the lower
division. The _Polypterus_ of the Nile and _Calamoichthys_ of South
Africa are the modern exemplars of this group. _Cheirolepis_, found in
the Old Red of Scotland and Canada, is the only Devonian
representative of the actinopterygian fishes. The cyclostome fishes
have, so far, been discovered only in Scotland, in the tiny
_Palaeospondylus_. Amphibian remains have been found in the Devonian
of Belgium; and footprints supposed to belong to a creature of the
same class (_Thinopus antiquus_) have been described by Professor
Marsh from the Chemung formation of Pennsylvania.

_Plant Life._--In the lacustrine deposits of the Old Red Sandstone we
find the earliest well-defined assemblage of terrestrial plants. In
some regions so abundant are the vegetable remains that in places they
form thin seams of veritable coal. These plants evidently flourished
around the shores of the lakes and lagoons in which their remains were
buried along with the other forms of life. Lycopods and ferns were the
predominant types; and it is important to notice that both groups were
already highly developed. The ferns include the genera _Sphenopteris_,
_Megalopteris_, _Archaeopteris_, _Neuropteris_. Among the Lycopods are
_Lycopodites_, _Psilophyton_, _Lepidodendron_. Modern horsetails are
represented by _Calamocladus_, _Asterocalamites_, _Annularia_. Of
great interest are the genera _Cordaites_, _Araucarioxylon_, &c.,
which were synthetic types, uniting in some degree the Coniferae and
t