Vertebrates.  It is with the greatest interest that we turn now to study the backboned animals of the Devonian; for they are believed to be the ancestors of the hosts of vertebrates which have since dominated the earth.  Their rudimentary structures foreshadowed what their descendants were to be, and give some clue to the earliest vertebrates from which they sprang.  Like those whose remains are found in the lower Paleozoic systems, all of these Devonian vertebrates were aquatic and go under the general designation of fishes.

The lowest in grade and nearest, perhaps, to the ancestral type of vertebrates, was the problematic creature, an inch or so long, of Figure 297.  Note the circular mouth not supplied with jaws, the lack of paired fins, and the symmetric tail fin, with the column of cartilaginous, ringlike vertebrae running through it to the end.  The animal is probably to be placed with the jawless lampreys and hags,—­a group too low to be included among true fishes.

OSTRACODERMS. This archaic group, long since extinct, is also too lowly to rank among the true fishes, for its members have neither jaws nor paired fins.  These small, fishlike forms were cased in front with bony plates developed in the skin and covered in the rear with scales.  The vertebrae were not ossified, for no trace of them has been found.

Devonian fishes.  The true fishes of the Devonian can best be understood by reference to their descendants now living.  Modern fishes are divided into several groups:  Sharks and their allies; dipnoans; ganoids, such as the sturgeon and gar; and teleosts,—­ most common fishes, such as the perch and cod.

Sharks.  Of all groups of living fishes the sharks are the oldest and still retain most fully the embryonic characters of their Paleozoic ancestors.  Such characters are the cartilaginous skeleton, and the separate gill slits with which the throat wall is pierced and which are arranged in line like the gill openings of the lamprey.  The sharks of the Silurian and Devonian are known to us chiefly by their teeth and fin spines, for they were unprotected by scales or plates, and were devoid of a bony skeleton.  Figure 299 is a restoration of an archaic shark from a somewhat higher horizon.  Note the seven gill slits and the lappetlike paired fins.  These fins seem to be remnants of the continuous fold of skin which, as embryology teaches, passed from fore to aft down each side of the primitive vertebrate.

Devonian sharks were comparatively small.  They had not evolved into the ferocious monsters which were later to be masters of the seas.