to those of the Oölitic formation, which immediately precedes, than to those of the carboniferous formation, which is much more ancient; and, in the same manner, the fossils of the carboniferous group approach more nearly to those of the Silurian formation than to those of the Tertiary.

468. These relations could not escape the observation of naturalists, and indeed they are of great importance for the true understanding of the development of life at the surface of our earth. And, as in the history of man, several grand periods have been established, under the name of Ages, marked by peculiarities in his social and intellectual condition, and illustrated by contemporaneous monuments, so, in the history of the earth, also, are distinguished several great periods, which may be designated as the various Ages of Nature, illustrated, in like manner, by their monuments, the fossil remains, which, by certain general traits stamped upon them, clearly indicate the eras to which they belong.

469. We distinguish four Ages of Nature, corresponding to the great geological divisions, namely:

1st. The Primary or Palæozoic Age, comprising the lower Silurian, the upper Silurian, and the Devonian. During this age there were no air-breathing animals. The fishes were the masters of creation. We may therefore call it the Reign of Fishes.

2d. The Secondary Age, comprising the carboniferous formation, the Trias, the Oölitic, and the Cretaceous formations. This is the epoch in which air-breathing animals first appear. Reptiles predominate over the other classes, and we may therefore call it the Reign of Reptiles.

3d. The Tertiary Age, comprising the tertiary formations. During this age, terrestrial mammals, of great size, abound. This is the Reign of Mammals.

4th. The Modern Age, characterized by the appearance of the most perfect of all created beings. This is the Reign of Man.

Let us review each of these four Ages of Nature, with reference to the diagram at the beginning of the volume.

470. THE PALEOZOIC AGE. Reign of Fishes. The palæozoic fauna, being the most remote from the present epoch, presents the least resemblance to the animals now existing, as will easily be perceived by a glance at the fol.

lowing sketches, (Fig. 155.) In no other case do we meet with animals of such extraordinary shapes, as in the strata of the Palæozoic age.

471. We have already stated (466) that there are found, in each formation of the primary age, animal remains of all the four great departments, namely, vertebrates, articulata, mollusks, and radiata. We have now to examine to what peculiar classes and families of each department these remains belong, with a view o ascertain if any relation between

the structure of an animal, and the epoch of its first appear ance on the earth's surface, may be traced.

472. As a general result of the inquiries hitherto made, it may be stated that the palæozoic animals belong, for the most part, to the lower divisions of the different classes. Thus, of the class of Echinoderms, we find scarcely any but Crinoids, which are the least perfect of the class. We have represented, in the above sketches, several of the most curious forms, as well as of the Polyps, of which there are some quite peculiar types from the Trenton limestone, and from the Black River limestone.

473. Of the Mollusks, the bivalves or Acephala are numerous, but, for the most part, they belong to the Brachiopoda, that is to say, to the lowest division of the class, including mollusks with unequal valves, having peculiar appendages in the interior. The Leptana alternata, (b,) which is found very abundantly in the Trenton limestone, is one of these shells. The only fossils yet found in the Potsdam sandstone, the oldest of all fossiliferous deposits, belong, also, to this family, (Lingula prima, a.) Besides this, there are also found some bivalves of a less uncommon shape, (Avicula decussata, e.)

474. The Gasteropods are less abundant; some of them are of a peculiar shape and structure, (Bucania expansa, f; Euomphalus hemisphericus, c.) Those more similar to our common marine snails have all an entire aperture; those with a canal being of a more recent epoch.

475. Of the Cephalopods we find some genera not less curious, part of which disappear in the succeeding epochs;

(i) Cyathocrinus ornatissimus, Hall; (j) Melocrinus Amphora, Goldf.; (k) Cariocrinus ornatus, Say; (1) Columnaria alveolata; (m) Cyathophyllum quadrigeminum, Goldf.; (n, o) Caninia flexuosa; (p) Chætetes lycoperdon.

such, in particular, as those of the straight, chambered shells called Orthoceratites, some of which are twelve feet in length, (Orthoceras fusiforme, g.) There are also found some of a coiled shape, like the Ammonites of the secondary age, but having less complicated partitions, (Trocholites ammonius, d.) The true cuttle-fishes, which are the highest of the class, are not yet found. On the contrary, the Bryozoa, which have long been considered as polyps, but which, according to all appearances, are mollusks of a very low order, are very numerous in this epoch.

476. The Articulata of the Palæozoic age are mostly Trilobites, animals which evidently belong to the lower order of the Crustaceans, (Fig. 156.) There is an incompleteness and want of development, in the form of their body, that strongly reminds us of the embryo among the crabs. A great many genera have already been discovered.

We may consider as belonging to the more extraordinary the forms here represented, (Harpes, a; Arges, b; Brontes, c; and Platynotus, d;) the latter, as well as the Isotelus, the largest of all, being peculiar to the Paleozoic deposit of this country. Some others seem more allied to the crusta ceans of the following ages, but are nevertheless of a very extraordinary form, as Eurypterus remipes, (e.) There are a.so found, in the Devonian, some very large Entomostraca. The class of Worms is represented only by a few Serpulæ,

which are marine worms, surrounded by a solid sheath. The class of Insects is entirely wanting.

477. The inferiority of the earliest inhabitants of our earth appears most striking among the Vertebrates. There are as yet neither reptiles, birds, nor mammals. The fishes, as we have said, are the sole representatives of this division of animals.

478. But the fishes of that early period were not like ours. Some of them had the most extraordinary forms, so that they have been often mistaken for quite different animals; for example, the Pterichthys, (a,) with its two wing

like appendages, and also the Coccosteus (b) of the same deposit, with its large plates covering the head and the anterior part of the body. There are also found remains of shark's spines, (e,) as well as palatal bones, (d,) the latter of a very peculiar kind. Even those fishes which have a more regular shape, as the Dipterus, (c,) have not horny scales like our common fishes, but are protected by a coat of bony plates, covered with enamel, like the gar-pikes of the American rivers. Moreover, they all exhibit certain characteristic features, which are very interesting in a physiological point of view. They all have a broad head, and a tail terminating in two unequal lobes. What is still more curious, the best preserved specimens show no indications