Page:Popular Science Monthly Volume 60.djvu/566

558 a word once used in the widest fashion for all sorts of mailed fishes, but little by little restricted to the hard-scaled relatives and ancestors of the gar-pike of to-day.

Dimly seen in the vast darkness of Paleozoic time are the huge figures known as Arthrognaths. These are mailed and helmeted fishes, limbless so far as we know, but with sharp, notched, turtle-like jaws quite different from those of the fish or those of any animal alive to-day. These creatures appear in Silurian rocks and are especially abundant in the fossil beds of Ohio, where Newberry, Claypole, Eastman and Dean have patiently studied the broken fragments of their armor. Most of them have a great casque on the head with a shield at the neck and a movable joint connecting the two. Among them was almost every variation in size and form.

These creatures have been often called ganoids, but with the true ganoids like the gar-pike they have seemingly nothing in common. They are very different from the Ostracophores. To regard them as derived from ancestral Dipnoi is to give a possible guess as to their origin and a very unsatisfactory guess at that. But they have all passed away in competition with the scaly fishes and sharks of later evolution, and it seems certain that they, like the mailed Ostracophores, have left no descendants.

Next after the lampreys, but a long way after in structure as in time, come the sharks. With the sharks appear for the first time true limbs and the lower jaw. The upper jaw is still formed from the palate, and the shoulder girdle is attached behind the skull. "Little is known," says Professor Dean, "of the primitive stem of the sharks and even the lines of descent of the different members of the group can only be generally suggested. The development of recent forms has yielded few results of undoubted value to the phylogenist. It would appear is if paleontology alone could solve the puzzles of their descent."

Of the very earliest sharks in the Upper Silurian age the remains are too scanty to prove much save that there were sharks in abundance and variety. Spines, teeth, fragments of shagreen, show that in some regards these forms were highly specialized. In the Carboniferous age, according to Dean, 'there occurred the culminating point of their differentiation when specialized sharks existed, whose varied structures are paralleled only by those of the existing bony fishes,—sharks fitted to the most special environment, some minute and delicate, others enormous, heavy and sluggish, with stout head and fin spines and elaborate types of dentition.'

The sharks are, however, doubtless evolved from the still more primitive shark without limbs and with the teeth slowly formed from modification of the ordinary shagreen prickles. In determining the earliest among the several primitive types of shark we are stopped by