=Invertebrates.= We must pass over the ever-changing groups of the invertebrates with the briefest notice. Chain corals became extinct at the close of the Silurian, but other corals were extremely common in the Devonian seas. At many places corals formed thin reefs, as at Louisville, Kentucky, where the hardness of the reef rock is one of the causes of the Falls of the Ohio. Sponges, echinoderms, brachiopods, and mollusks were abundant. The cephalopods take a new departure. So far in all their various forms, whether straight, as the Orthoceras, or curved, or close-coiled as in the nautilus, the septum, or partition dividing the chambers, met the inner shell along a simple line, like that of the rim of a saucer. There now begins a growth of the septum by which its edges become sharply corrugated, and the suture, or line of juncture of the septum and the shell, is thus angled. The group in which this growth of the septum takes place is called the _Goniatite_ (Greek _gonia_, angle). [Illustration: Fig. 296. A Goniatite] =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 vertebræ 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. [Illustration: Fig. 297. Palæospondylus] =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 vertebræ were not ossified, for no trace of them has been found. [Illustration: 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. [Illustration: Fig. 299. A Paleozoic Shark]