The lithophytes, including many of the corals, as they are commonly called, together with the more complicated crustaceous polypi, form an important branch in the first division. In the coral islands of the Pacific Ocean, many species are found, unmixed with common stony or earthy matter, forming immense masses ; and in many places where they are plainly of a far higher antiquity, they also constitute entire strata ; coalescing into a solid rock by means of calcareous matter which has probably been furnished from their own ruins. In other instances, the fossils of this divisions are sparingly interspersed in the calcareous beds; either alone, or intermixed with other shells, or with remains of various kinds. In a few cases only they have been found dispersed among alluvial mat ters; and, in these they are either the consequence of the decomposition of rocks in which they have been imbedded, or else they belong to those peculiar deposits formerly mentioned, that arc the consequence either of the recent partial retreat of the sea, or of the elevation of the land.
The shells of testaceous animals form the next, and by far the most abundant, division of fossil bodies, whether we regard their absolute quantity or the number and va riety of the species. These present the same varieties in the modes in which they are found; but as they are more numerous than the preceding tribes, so they occupy a more extensive range in the series of the stratified rocks.
The remains of crustaceous insects are far more rare, and they are indeed extremely limited; whether we regard their absolute numbers, the strata in which they are found, or the number of species which they present. The same remark may be made on fishes; as their places among rocks are limited to two or three, and as they abound chiefly in situations where they appear to have been sud denly elevated out of the sea by revolutions of compara tively recent date. It is not difficult to understand why they should be thus rare; since, from their perishable na ture, the various circumstances required for their preser vation could not often have been present. Accordingly, their harder and more durable remains arc often found where no other testimonials of their former existence are present; indicating that they may probably have abounded in the waters of the most ancient ocean, although we are unable to produce any proofs of the fact.
The remains of the larger fish, or the cetacea, have also been found in a fossil state, yet rarely ; and the most re markable instances of this nature are where they exist in alluvial soils, as in those of Italy, where they have been elevated from the sea by recent revolutions of the surface ; or in other situations, such as that near Stirling in our own country, where they have been entangled among those al luvia of rivers that have, by their gradual accumulation, excluded the ocean and generated tracts of dry land.
Few amphibious animals have been found among those rocks that are strictly of marine origin, although there are some obscurities respecting this subject that have not yet been cleared up. In the chalk, remains of tortoises are
known to exist. In the lias limestone, a stratum high ur in the series, though far beneath the chalk, there have been found the remains of crocodiles ; animals which, if they had the same habits as those of the present day, must have been of fresh water rather than of marine origin ; although indeed it does appear, from the remarks of some of our navigators, that crocodiles frequenting the sea arc found in the Palaos islands. But the origin of the lias, in particular places, is rendered some what obscure by its con taining also the remains of vegetables ; so that it must still remain a doubt under what circumstances this strange but important series has been produced, and consequently, what are the real natures of many of the organic substan ces which it includes.
It was just remarked that marine fossils did not often comprise vegetable remains. It should rather be said that no very unexceptionable instance of a marine plant im bedded in a rock has been produced. But the fragments of terrestrial plants are sometimes found intermixed with fossils of marine origin in the same deposits, although this happens in a few rare instances only ; in the coal strata, for example, and in those strata of recent origin that appear to have been formed from the joint or alternate repose of fresh and of salt water. These remains, however, must be considered as of a terrestrial nature; and whatever difficul ties their present connections may offer, they are explain ed, as well as our geological knowledge of these particular deposits at present admits, in the history of these alternat ing formations.
The terrestrial fossils, which are connected with fresh water, or which are more or less completely of an aquatic nature, include shells, lithophytes, fishes, amphibious ani mals, and plants. The shells of the fresh-water testacea are found under various circumstances similar to those be longing to the marine tribes ; but they are, of course, com paratively very limited, and even more so in their absolute numbers than in that of the species. The rocks of which they form portions, or into which they enter, belong prin cipally to the coal series and to the fresh-water deposits, as might be expected ; although, since the remains of amphi bious animals and of vegetables occur in the more general strata that lie above the coal series, as in the lias lime stone, it is not to be wondered at if fresh-water shells should exist there also, however difficult it may be to ex plain that fact. Milleporx and echini have also been ob served among the coal strata; but as in many of these there are obvious marks of a mixed terrestrial and marine origin, it must remain a doubt whether these animals do not belong to the latter rather than to the former part of the deposits.