kinds preserved which are wholly destitute of hard parts. I do not attempt any description of the Polyps proper, because the early Corals of that class are comparatively few, and do not present features sufficiently characteristic to attract the notice of the casual observer.

Of the Echinoderms, the class of Radiates represented now by our Star-Fishes and Sea-Urchins, we may gather any quantity, though the old-fashioned forms are very different from the living ones. I have dwelt at such length in a former article on the wonderful beauty and variety of the Crinoids, or " Stone Lilies," as they have been called, from their resemblance to flowers, that I will only briefly allude to them here. The subjoined wood-cut represents one with a closed

cup; but the number of their different patterns is hardly to be counted, and I would invite any one who questions the abundant expression of life in those days to look at some slabs of ancient limestone in the Zoological Museum at Cambridge, where the stems of the Crinoids are tangled together as thickly as sea-weed on the shore. Indeed, some of our rock-deposits consist chiefly of the fragments of their remains.

The Mollusks were also represented then, as now, by their three classes,

*See Methods of Study in Natural History, Atlantic Monthly, No. LVII., July, 1862.

Acephala, Gasteropoda, and Cephalopoda. The Acephala or Bivalves we shall find in great numbers, but of a very different pattern from the Oysters, Clams, and Mussels of recent times. The annexed wood-cut represents one of these

Brachiopods, which form a very characteristic type of the Silurian deposits. The square cut of the upper edge, where the two valves meet along the back and are united by a hinge, is altogether oldfashioned, and unknown among our modern Bivalves. The wood-cut does not show the inequality of the two valves, also a very characteristic feature of this group, one valve being flat and fitting closely into the other, which is more spreading and much fuller. These, also, were represented by a great variety of species, and we find them crowded together as closely in the ancient rocks as Oysters or Clams or Mussels on any of our modern shores. Besides these, there were the Bryozoa, a small kind of Mollusk allied to the Clams, and very busy then in the ancient Coral work. They grew in communities, and the separate individuals are so minute that a Bryozoan stock looks like some delicate moss. They still have their place among the Reef-Building Corals, but play an insignificant part in comparison with that of their predecessors.

Of the Silurian Univalves or Gasteropods there is not much to tell, for their spiral shells were so brittle that scarcely any perfect specimens are known, though their broken remains are found in such quantities as to show that this class also was very fully represented in the earliest creation. But the highest class of Mollusks, the Cephalopods or Chambered Shells, or Cuttle-Fishes, as they are called when the animal is unprotected by a shell, are, on the contrary, very

well preserved, and they are very numerous. Of these I will speak somewhat more in detail, because their geological history is a very curious one.

The Chambered Nautilus is familiar to all, since, from the exquisite beauty of its shell, it is especially sought for by conchologists; but it is nevertheless not so common in our days as the Squids and Cuttle-Fishes, which are the most numerous modern representatives of the class. In the earliest geological days, on the contrary, those with a shell predominated, differing from the later ones, however, in having the shell perfectly straight instead of curved, though its internal structure was the same as it is now and has ever been. Then, as now, the animal shut himself out from his last year's home, building his annual wall behind him, till his whole shell was divided into successive chambers, all of which were connect-. ed by a siphon. Some of the shells of this kind belonging to the Silurian deposits are enormous: giants of the sea they must have been in those days. They have been found fifteen feet long, and as large round as a man's body. One can imagine that the Cuttle-Fish inhabiting such a shell must have been a formidable animal. These straight-chambered shells of the Silurian and Devonian seas are called Orthoceratites (see wood-cut below).

We shall meet them again hereafter, under another name and with a different form; for, as they advance in the geological ages, they not only assume the curved outline with ever closer whorls till it culminates in the compact coil of the Ammonites of the middle periods, but the partitions, which are perfectly plain walls in these earlier forms, become scalloped and involuted along the edges in the later ones, making the most delicate and exquisite tracery on the surface of the shell.

Of Articulates we find only two classes, Worms and Crustacea. Insects there were none, for, as we have seen, this early world was wholly marine. There is little to be said of the Worms, for their soft bodies, unprotected by any hard covering, could hardly be preserved; but, like the marine Worms of our own times, they were in the habit of constructing envelopes for themselves, built of sand, or sometimes from a secretion of their own bodies, and these cases we find in the earliest deposits, giving us assurance that the Worms were represented there. I should add, however, that many impressions described as produced by Worms are more likely to have been the tracks of Crustacea.

But by far the most characteristic class of Articulates in ancient times were the Crustaceans. The Trilobites stand in the same relation to the modern Crustacea as the Crinoids do to the modern Echinoderms. They were then the sole representatives of the class, and the variety and richness of the type are most extraordinary. They were of nearly equal breadth for the whole length of the body, and rounded at the two ends, so as to form an oval outline. To give any adequate idea of the number and variety of species would fill a volume, but I may enumerate some of the more striking differences: as, for instance, the greater or less prominence of the anterior shield, the preponderance of the posterior end in some, while in others the two ends are nearly equal, the presence or absence of prongs on the shield and of spines along the sides of the body,

appendages on the head in some species, of which others are entirely destitute, and the smooth outline of some, while in others the surface is broken by a variety of external ornamentation. Such are a few of the more prominent differences among them. But the general structural features are the same in all. The middle region of the body is always divided in uniform rings, lobed in the middle so as to make a ridge along the back with a slight depression on either side of it. It is from this three-lobed division that they receive their name. The subjoined wood-cut represents a characteristic Silurian Trilobite.

There is no group more prominent in the earliest creations than this one of the Trilobites, and so exclusively do they belong to them, that, as we shall see, in proportion as the later representatives of the class come in, these old-world Crustaceans drop out of the ranks, fall behind, as it were, in the long procession of animals, and are left in the ancient deposits. Even in the Carboniferous period but few are to be found: they had their day in the Silurian and Devonian ages. In consequence of their solid exterior, the preservation of these animals is very complete; and their attitudes are often so natural, and the condition of all their parts so perfect, that one would say they had died yesterday rather than countless centuries ago.

Their geological history has been very thoroughly studied; not only are we fa

iliar with all their adult characters, but

even their embryology is well known to naturalists. It is, indeed, wonderful that the mode of growth of animals which died out in the Carboniferous period should be better known to us than that of many living types. But it is nevertheless true that their embryonic forms have been found perfectly preserved in the rocks, and Barrande, in his "Système Silurien de la Bohème," gives us all the stages of their development, from the time when the animal is merely sketched out as a simple furrow in the embryo to its mature condition. So complete is the sequence, that the plate on which their embryonic changes are illustrated contains more than thirty figures, all representing different phases of their growth. There is not a living Crab represented so fully in any of our scientific works as is that one species of Trilobite whose whole story Barrande has traced from the egg to its adult size. Such facts should make those who rest their fanciful theories of the origin and development of life on the imperfection of the geological record, filling up the supposed lapses to suit themselves, more cautious as to their results.

We have found, then, Radiates, Mollusks, and Articulates in plenty; and now what is to be said of Vertebrates in these old times, - of the highest and most important division of the Animal Kingdom, that to which we ourselves belong? They were represented by Fishes alone; and the Fish chapter in the history of the early organic world is a curious, and, as it seems to me, a very significant one. We shall find no perfect specimens; and he would be a daring, not to say a presumptuous thinker, who would venture to reconstruct a fish of the Silurian age from any remains that are left to us. But still we find enough to indicate clearly the style of those old fishes, and to show, by comparison with the living types, to what group of modern times they belong. We should naturally expect to find the Vertebrates introduced in their simplest form; but this is by no means the case: the common fishes, as Cod, Herring, Mack

ters.

But there are two groups of so-called fishes, differing from these by some marked features, among which we may find the modern representatives of these earliest Vertebrates. Of these two groups one consists chiefly now of the Gar-Pikes of our Western waters, though the Sturgeons share also in some of their features. In these fishes there is a singular union of reptilian with fish-like characThe systems of circulation and of respiration in them are more complicated than in the common fishes; the structure of the skull resembles that of the skull in reptiles, and they have other reptilian characters, such as their ability to move the head upon the neck independently of the body, and the connection of the vertebræ by ball-and-socket joint, instead of by inverted cones, as in the ordinary fishes. Their scales are also peculiar, being covered by enamel so hard, that, if struck with steel, they will emit sparks like flint. It is on account of this peculiarity that the whole group has been called Ganoid. Now, though we have not found as yet any complete specimens of Silurian fishes, their disconnected remains are scattered profusely in the early deposits. The scales, parts of the backbone, parts of the skull, the teeth, are found in a tolerable state of preservation; and these indications, fragmentary as they are, give us the clue to the character of the most ancient fishes. A large proportion of them were no doubt Ganoids; for they had the same peculiar articulation of the vertebræ, the flexibility of the neck, and the hard scales so characteristic of our Gar-Pikes.

There is another type of these ancient Vertebrates, which has also its represent atives among our modern fishes. These are the Sharks and Skates, or, as the Greeks used to call them, the Selachians, -making a very appropriate distinction between them and common fishes, on account of the difference in the structure of the skeleton. In Selachians the quality of the bones is granular, instead of

below the backbone are formed by flat plates, instead of the spines so characteristic of all the fish proper; and the skull consists of a solid box, instead of being built of overlapping pieces like the true fish-skull. They differ also in their teeth, which, instead of being implanted in the bone by a root, as in fishes, are loosely set in the gum without any connection with the bone, and are movable, being arranged in several rows one behind another, the back rows moving forward to take the place of the front ones when the latter are worn off. They are unlike the common fishes also in having the backbone continued to the very end of the tail, which is cut in uneven lobes, the upper lobe being the longer of the two, while the terminal fin, so constant a feature in fishes, is wanting. The Selachians resemble higher Vertebrate types not only in the small number of their eggs, and in the closer connection of the young with the mother, but also in their embryological development, which has many features in common with that of birds and turtles. Of this group, also, we find numerous remains in the ancient geological deposits; and though we have not the means of distinguishing the species, we have ample evidence for determining the type.

This combination of higher with lower features in the earlier organic forms is very striking, and becomes still more significant when we find that many of the later types recall the more ancient ones. I have called these more comprehensive groups of former times, combining characters of different classes, synthetic or prophetic types; and we might as fitly give the name of retrospective types to many of the later groups, for they recall the past, as the former anticipate the future. And it is not only among the Fishes and the Reptiles that we find these combinations. The most numerous of the ancient Radiates are the Acalephian Corals, combining, in the Hydroid form, the Polyp-like mode of life, habits, and general appearance with

the structure of Acalephs. The Crinoids, with the closed cups in some, and the open, star-like crowns in others, unite features of the present Star-Fishes and Sea-Urchins, and, by their stem attaching them to the ground, include also a Polyp-like character; while the Trilobites, with their uniform rings and their prominent anterior shield, unite characters of Worms and Crustacea.

These early types seem to sketch in broad, general characters the Creative purpose, and to include in the first average expression of the plan all its structural possibilities. The Crinoid forms include the thought of the modern StarFishes and Sea - Urchins; the simple chambered shells of the Silurian anticipate the more complicated structure of the later ones; the Trilobites give the most comprehensive expression of the Articulate type; while the early Fishes not only prophesy the Reptiles which are to come, but also hint at Birds and even at Mammalia by their embryonic development and their mode of reproduction.

Looked at from this point of view, the animal world is an intellectual Creation, complete in all its parts, and coherent throughout; and when we find, that, although these ancient types have become obsolete and been replaced by modern ones, yet there are always a few oldfashioned individuals, left behind, as it were, to give the key to the history of their race, as the Gar-Pike, for instance, to explain the ancient Fishes, the Millepore to explain the old Acalephian Corals, the Nautilus to be the modern exponent of the Ammonites and Orthoceratites of past times, we cannot avoid the impression that this Creative work has been intended also to be educational for Man, and to teach him his own relation to the organic world. The embryology of the modern types confirms this idea, for here we find an epitome of their geological history. The embryo of the present Star - Fishes recalls the Crinoids; the embryo of the Crab recalls the Trilobites; the embryo of the Vertebrates,

including even that of the higher Mammalia, recalls the ancient Fishes. Does not this fact, that the individual animal in its growth recalls the history of its type, prove that the Creative Thought in its immediate present action embraces all that has gone before, as its first organic expression included all that was to come? The study of Nature in its highest meaning shows us the present doubly rich with all the past, and the past linked and interwoven with the present, not lying divorced and dead behind it.

I have spoken of the Silurian beach as if there were but one, not only because I wished to limit my sketch, and to attempt at least to give it the vividness of a special locality, but also because a single such shore will give us as good an idea of the characteristic fauna of the time as if we drew our material from a wider range. There are, however, a great number of parallel ridges belonging to the Silurian and Devonian periods, running from east to west, not only through the State of New York, but far beyond, through the States of Michigan and Wisconsin into Minnesota; one may follow nine or ten such successive shores in unbroken lines, from the neighborhood of Lake Champlain to the Far West. They have all the irregularities of modern sea-shores, running up to form little bays here, and jutting out in promontories there; and upon each one are found animals of the same kind, but differing in species from those of the preceding.

Although the early geological periods are more legible in North America, because they are exposed over such extensive tracts of land, yet they have been studied in many other parts of the globe. In Norway, in Germany, in France, in Russia, in Siberia, in Kamtchatka, in parts of South America, in short, wherever the civilization of the white race has extended, Silurian deposits have been observed, and everywhere they bear the same testimony to a profuse and varied creation. The earth was teeming then with life as now, and in whatever corner of its surface the geolo