Fig. 298. An Ostracoderm
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.
Fig. 299. A Paleozoic Shark
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.
Fig. 300. A Devonian Dipnoan