itself and exposes a large surface beyond the aperture of the shell, and at the same time produces a slight vacuum in the last chamber, its specific gravity becomes on the other hand a little less than that of water, and it rises rapidly to the surface. It may also be the case that the curious tube, or siphuncle, that runs through all the chambers, assists in some way in thus adjusting the balance of the animal; although, from the appearance of this tube, coated with a thin calcareous deposit, it seems unlikely that its dilatation or contraction could produce any useful effect.* Contemporaneous with the various groups of animals Figs. 14 15 already described, there seem to have been introduced in the primæval seas a large number of species very closely allied to the nautilus, but provided with floats or chambered shells not coiled into a spiral as is the case with the recent analogue, but either straight or very slightly curved; and from their resemblance to a horn, called by naturalists Orthocerast (or straight horn), Cyrtoceras or (bent horn), &c. The animals inhabiting these shells must no doubt have been very closely allied to the recent nautilus; but nothing is known of them except the fragments of their habitations, which exhibit great variety of form ORTHOCERATITES. and some rather incomprehensible *Owen's Lectures on Comp. Anat., p. 327 et seq. + Oplos (orthos), straight; kupτos (cyrtus), curved; and кɛpaç (ceras), a horn. peculiarities of structure. Some of them appear to have been of great length and exceedingly slender; and the shell is often thin, although in that case the walls of separation between each two successive chambers are generally close together. The sides of the shell are often deeply ribbed or grooved, sometimes in the direction of its length, and sometimes across. A few of the species had their shells short and greatly swelled, having almost the shape of a pear; some again came rapidly to a pointed termination; and some were so nearly cylindrical, that it is difficult to suppose that they ever commenced at a point, and were increased by regular gradations. All these varieties of form are met with in the oldest rocks; and the large and important group of Orthoceratites, apparently the first, as it is the simplest form of the multilocular shell, seems to have attained its greatest development very early, and then was gradually replaced by other groups of Cephalopoda; until, towards the close of the first epoch, these animals had died out entirely, and were replaced by the nautilus, and yet more remarkably by the ammonites, which then appeared for the first time. We have now gone through the description of the inhabitants of the seas during the earliest period at which the Geologist is enabled to trace the existence of living beings upon the earth. Let us, before concluding, briefly reconsider the results of geological investigation with regard to these ancient strata. In the first place, it is interesting to remark, that, among the groups exhibiting the lowest amount of organization, there are a few corallines, and a larger number of the stony corals. There are also several species of crinoids (lily-shaped polyps), individually abundant; and although there appear to be one or two species of the more highly organized Radiata (such as the star-fish), we very rarely find remains of other radiated animals than encrinites in the old rocks, although numerous higher forms afterwards became exceedingly abundant. Of the crustaceans again we obtain no fragments of true crabs or lobsters, or other common and known forms, but instead of them a group, long since extinct, not more likely to be preserved than the former, and, although for a time evidently very common, not continued into the middle one of the three great periods. The absence or rarity of the common bivalve and univalve shells in these rocks is also a point of very considerable interest. A few species of the family represented by the common cockle (Cardium), and a few also of the scallop tribe (Pecten, Avicula), both of which groups are remarkable among the shell-bearing animals for their locomotive powers, and the extent to which they adapt themselves to changing circumstances, are among the chief of the bivalves; and a number of species nearly allied to the carnivorous Buccinum, or whelk, represented in like manner the univalves. But although the genera now common were then so rare, their place was evidently supplied by two other groups now nearly lost sight of; and of these the vast number of shells allied to Terebratula, and the abundance of Orthoceratites, form the most striking and valuable examples. Quite at the close of the period we are considering, a few small fishes, apparently allied to the shark tribe, were also introduced as typical forms of what should afterwards abound. Although, however, these fishes were introduced towards the close but before the termination of the period of the Invertebrata, it is important to remember that almost all the great natural divisions of the Invertebrata began at once and together to perform their work on earth; so that there is no appearance of any regular order of progression by which the encrinite succeeded the coral polyp, the trilobite the encrinite, the terebratula the trilobite, or the orthoceratite the brachiopod. All these seem to have been truly contemporaneous, and they were doubtless introduced as the group best fitted to perform the functions of their existence during the conditions, whatever they may have been, under which the world existed in their time. And so little in many points do the differences of their organization seem to require important changes in the temperature and atmospheric condition of the earth, so similar are the living species most nearly allied to them in all peculiarities of which we can fairly judge, that, however we may be inclined to conjecture and speculate on the probability of a higher and more uniform temperature, a more widely extended sea receiving similar deposits and containing similar species, or an atmosphere more highly charged with carbonic acid gas than at present, these speculations must be kept within bounds, since the facts justify no more than the admission of their bare possibility. The mere absence of certain groups or of certain species and genera afterwards common, is not the only point on which the naturalist dwells in considering the possible conditions of the ancient sea. It is much more important, and much more interest ing, to observe that these species were not only absent, but that their place was supplied by other groups of animals analogous to them, having similar habits, but not identical in specific character. Thus the place of the less highly organized of the common shell-fish, such as the muscles, the oyster tribe, and the like, was properly filled by numerous and varied forms allied to Terebratula (a lower group); while the numerous groups of flesh-eating Gasteropoda (the Murex, the cone, the volute, the cowry, and many others) were equally well represented by innumerable orthoceratites (animals of higher organization), which then swarmed in the seas. No doubt the appearance of these ancient seas would have appeared strange to the eyes of the naturalist, could an inhabitant of the world in its present state have become acquainted with the mysteries of the ocean's deep abysses at that time. With something of resemblance in the reefs and islands of coral rising gradually to the water's edge, as the coral polyp toiled and laboured from day to day and from year to year, there would yet be much more of difference both in the shallows and depths of the ocean. The former sometimes with a sandy, but more frequently a muddy bottom, would be peopled with countless myriads of those unsightly animals, the trilobites, swimming near the surface of the water with their backs downwards, looking out constantly, and sinking at the slightest approach of danger from beneath; while the remains of successive generations of these creatures, mixed with mud and sand, would rapidly form beds sometimes of great extent. From amongst such beds, or attached to the solid rock, would be |