Page:Popular Science Monthly Volume 12.djvu/533

Rh laid down, this class was well represented. During the remainder of the Devonian, fishes continue abundant in the shallower seas, and, so far as now known, were the only type of vertebrate life. These fishes were mainly Ganoids, a group represented in our present waters by the gar-pike (Lepidosteus) and Sturgeon (Acipenser), but, in the Devonian sea, chiefly by the Placoderms, the exact affinities of which are somewhat in doubt. With these were Elasmobranchs, or the Shark tribe, and among them a few Chimæroids, a peculiar type, of which one or two members still survive. The Placoderms were the monarchs of the ocean. All were well protected by a massive coat of armor, and some of them attained huge dimensions. The American Devonian fishes now known are not so numerous as those of Europe, but they were larger in size, and mostly inhabitants of the open sea. Some twenty genera and forty species have been described.

The more important genera of Placoderms are—Dinichthys, Aspidichthys, and Diplognathus, our largest Palæozoic fishes. Others are—Acanthaspis, Acantholepis, Coccosteus, Macropetalichthys, and Onychodus. Among the Elasmobranchs were—Cladodus, Ctenacanthus, Machœracanthus, Rhynchodus, and Ptyctodus, the last two being regarded as Chimæroids. In the Chemung epoch, the great Dipterian family was introduced with Dipterus, Heliodus, and possibly Ceratodus. Species of the European genera, Bothriolepis and Holoptychius, have likewise been found in our Devonian deposits.

With the close of the Devonian, came the almost total extinction of the great group of Placoderms, while the Elasmobranchs, which had hitherto occupied a subordinate position, increase in numbers and size, and appear to be represented by Sharks, Rays, and Chimæras. Among the members of this group from the Carboniferous, were numerous Cestracionts, species of Cochliodus of large size, with others of the genera Deltodus, Helodus, Psammodus, and Sandalodus. Of the Petalodonts there were Antliodus, Chomatodus, Ctenoptychius, Petalodus, and Petalorhynchus; and of the Hybodonts, the genera Cladodus, Carcharopsis, and Diplodus. These Elasmobranchs were the rulers of the Carboniferous open sea, and more than one hundred species have been found in the lower part of this formation alone. The Ganoids, although still abundant, were of smaller size, and denizens of the more shallow and confined waters. The latter group of fishes was represented by true Lepidostidæ, of the genera Palæaniscus, Amblypterus, Platysomus, and Eurylepis. Other genera are—Rhizodus, Megalichthys, Ctenodus, Edestus, Orodus, Ctenacanthus, Gyracanthus, and Cœlacanthus. Most of these genera occur also in Europe.

From the Permian rocks of America no vertebrate remains are