Fossil shell

“ FORNICATED. Arched or vaulted, as the exfoliations on the costæ of Tridacna Elongata, fig. 157.
FOSSIL SHELL. A shell is considered to be in a fossil state when, the soft parts having ceased to exist, it is deprived of all its animal juices, has lost all, or nearly all its natural colour, and is thus changed in its chemical composition, when little or nothing is left but a mere bone, which is embedded in a sedimentary deposit. ”

“ The first of the above terms, Eocene, is derived from ἠώς eōs, dawn, and καινός kainos, recent; because an extremely small proportion of the fossil shells of this period could be referred to living species, so that this era seemed to indicate the dawn of the present testaceous fauna, no living species of shells having been detected in the antecedent or secondary rocks.
Some conchologists are now unwilling to allow that any Eocene species of shell has really survived to our times so unaltered as to allow of its specific identification with a living species. ”

“ Some fossil shells, even if simply attached to the OUTSIDE of others, bear full testimony to the conclusion above alluded to, namely, that an interval elapsed between the death of the creature to whose shell they adhere, and the burial of the same in mud or sand. The sea-urchins, or Echini, so abundant in white chalk, afford a good illustration. It is well known that these animals, when living, are invariably covered with spines supported by rows of tubercles. ”

“ The fleshy parts of animals do not petrify.
(3) Many animals among the invertebrates use mineral substances for protective or supporting structures. Small plants of various kinds follow a similar practice. These structures, being produced in stony materials, are readily converted into fossils. The shells of mollusks are the best known illustrations in this field, and all that is required for a shell to become a fossil is the extinction of the species of animal that produced it. Fossils of this type are extremely abundant. ”

“ They exactly resemble fossil shells in situ, but the species are absolutely identical with those that crawl over the shrubs and trees in the same region. In very many cases the dead shells accurately fit the hollows in the rock, whose interior is impressed with the form and sculpturing of the shell in intaglio:—a most curious and interesting fact, as it points to the very recent formation of the region, the stone bearing evident tokens of having been in a plastic condition when the shells were enveloped in it. ”

“ These characters are distinctly observable in one of the specimens [1] which contains, with several other remains of these fossils, a nearly perfect shell, longitudinally divided, so as to display very beautifully the two ridges, with the numerous septa and chambers. In another, [2] a longitudinal section of one of these fossil shells, the same parts are shewn, and the extreme thinness of the septa, and the closeness with which they are placed to each other, are particularly observable. ”

“ The tertiary strata described in the preceding chapters are all of them characterized by fossil shells, of which a considerable proportion are specifically identical with the living mollusca; and the greater the number, the more nearly does the entire fauna approach in species and genera to that now inhabiting the adjoining seas. But in the Eocene formations next to be considered, the proportion of recent species is very small, and sometimes scarcely appreciable, and those agreeing with the fossil testacea often belong to remote parts of the globe, and to various zoological provinces. ”

“ Now the remains of animals, especially of aquatic species, are found almost everywhere imbedded in stratified rocks, and sometimes, in the case of limestone, they are in such abundance as to constitute the entire mass of the rock itself. Shells and corals are the most frequent, and with them are often associated the bones and teeth of fishes, fragments of wood, impressions of leaves, and other organic substances. Fossil shells, of forms such as now abound in the sea, are met with far inland, both near the surface, and at great depths below it. ”

“ Thus, in the earlier Tertiary period, we find about 95 per cent of the known fossil shells to be species that are no longer in existence, the remaining 5 per cent being forms which are known to live in our present seas. In the middle of the Tertiary period we find many more recent and still existing species of shells, and the extinct types are much fewer in number ”

“ Almost every where on land we meet with fossil-shells, and of those of the same kind, some are small, others large, some young, others old; some imperfect, others extremely perfect and we likewise sometimes see the young ones adhering to the old. ”

“ A scrupulous comparison of the forms of these remains, of their texture, and often even of their chemical composition, does not disclose the slightest difference between the fossil shells and those which still inhabit the sea: the preservation of the former is not less perfect than that of the latter; most commonly we neither observe [8] detrition nor fracture in them, nothing, in short, that announces a violent removal from their original places; the smallest of them retain their sharpest ridges, and their most delicate spines. ”

“ In France, the fossil shells of Courtagnon near Reims, Grignon near Versailles, of what was formerly Touraine, etc., are almost all still in this calcareous state, having more or less completely lost their animal parts—namely, their lustre, their peculiar colors, and their nacre.
“Other fossils have undergone such an alteration that not only have they lost their animal portion, but their substance has been changed into a silicious matter. ”

“ The beautiful prismatic colours seen on the Labrodore stone are owing to a similar cause, viz. the thinness of the laminae of which it consists, and has probably been formed from mother-pearl shells.
It is curious that some of the most common fossil shells are not now known in their recent state, as the cornua ammonis; and on the contrary, many shells which are very plentiful in their recent state, as limpets, sea-ears, volutes, cowries, are very rarely found fossil. ”

“ The total number of fossil marine mollusca already collected is about equal to that of the living races: what may be the proportions hereafter is difficult to estimate, for it is certain that great additions will be made to both the catalogues.
It is not entirely without reason that geologists have been long accustomed to look on the study of fossil shells as more instructive with regard to the physical conditions of the globe in ancient times than most other reliquiæ of animal life. ”

“ Except as shedding some light on the structure and economy of their inhabitants, the shells, in the estimation of the naturalist, are the least interesting part of the organization of the Mollusca; but to the geologist, from their permanent nature, and the proofs they yield of the conditions under which the strata that contain them were deposited, they are important in the highest degree. It has even been found convenient to classify formations, in which fossil shells largely prevail, by the relative numerical proportion of the recent and extinct species found in the different groups of strata ”

“ Lign. 109, fig. 1.—The shell is turbinated, the cells are spheroidal, and the last, or terminal one, has a semilunar aperture at the umbilical angle. Several fossil species abound in the Chalk and in the tertiary deposits; and many living species swarm in our seas. ”

“ Within a few years, conchology has become a study of considerable importance, from its close connexion with geology. Students in the latter science must be well acquainted with fossil-shells, because they form so large a portion of organic remains. Species of recent shells, or those still existing, are also often found in a fossil state, while many fossil genera are now totally unknown in our earth and waters. ”

“ It is very true that, of the great quantity of fossil shells gathered in the different countries of the earth, there are yet but a very small number of species whose living or marine analogues are known. ”

“ Fossil shells are usually found quite entire, and retaining all the characters requisite for comparing them with the specimens contained in collections of natural history, or represented in the works of naturalists. Even the skeletons of fishes are found more or less entire, so that the general forms of their bodies can, for the most part, be ascertained, and usually, at least, their generic and specific characters are determinable, as these are mostly drawn from their solid parts. ”

“ Such facts by no means invalidate the general rule, that certain fossils are good chronological tests of geological periods; but they serve to caution us against attaching too much importance to single species, some of which may have a wider, others a more confined vertical range. We have before seen that, in the successive tertiary formations, there are species common to older and newer groups, yet these groups are distinguishable from one another by a comparison of the whole assemblage of fossil shells proper to each. ”

“ It is well to remark that among the fossil shells whose marine or living analogues are not known, there are many which have a form closely allied to shells of the same genera known to be now living in the sea. However, they differ more or less, and cannot be rigorously regarded as the same species as those known to be living, since they do not perfectly resemble them. These are, it is said, extinct species. ”

“ From the extremity of the last segment, there issue, sometimes from one, sometimes from several openings of the shell, or through numerous pores or foramina, very elongated, slender, contractile, colourless filaments, more or less divided and ramified, serving for prehension, and capable of entirely investing the shell. The body varies in colour, but is always identical in individuals of the same species,—it is yellow, fawn-coloured, red, [ 187 ] violet, blue, &c. ”

“ No tribe of animals shows more distinctly than do the Molluscs, how very different the value of fossils is in geology and in phylogeny. In geology the different species of the fossil shells of Molluscs are of the greatest importance because they serve as excellent marks whereby to characterize the different groups of strata, and to fix their relative ages. ”

“ HAVING hitherto considered those formations in which both the fossil shells and the mammalia are of living species, we may now turn our attention to those of older date, in which the shells being all recent, some of the accompanying mammalia are extinct, or belong to species not known to have lived within the times of history or tradition. ”

“ Thus, at Navidad, the strata are about eight hundred feet in thickness, and the fossil shells are abundant both at the level of the sea and some way up the cliffs; having sent a list of these fossils to Professor E. Forbes, he thinks they must have lived in water between one and ten fathoms in depth: hence the bottom of the sea on which these shells once lived must have subsided at least 700 feet to allow of the superincumbent matter being deposited. ”

“ No fossils have contributed more to this kind of information, than those of shells, and as the mineralized species could not be satisfactorily studied, except by accurately comparing them with those which now inhabit our seas and continents; the search for shells, has become, from a simple amusement, the study of scientific men—or, as a writer remarks, "it was only after the period when it was perceived that geology and ancient zoology were destined to be enlightened by their fossil remains, that this research passed from the hands of amateurs into those of naturalists." ”

“ On this rock rests clay, varying in purity from the finest pipe-clay to ochre. In this alluvium are found silicious fossils of two species of shells, similar to those fossilized with aluminous earth in the strata of the same kind at Eagle Hawk Neck; particularly the butterfly shell, as it is called, upon the spot. The siliceous fossils of this genus are almost all imperfect, but do not bear marks of attrition: the contained animals are as completely fossilized as the shells. Their structure, which is curious, may perhaps be as fully demonstrated from these as by living specimens. ”

“ We may study genera and species in museums, but the Fauna of different ages, the predominance of certain shells, the numerical relations which characterize the animal kingdom and the vegetation of a place or of a period, should be studied in sight of those formations. It has long appeared to me that in the tropics as well as in the temperate zone the species of univalve shells are much more numerous than bivalves. From this superiority in number the organic fossil world furnishes, in every latitude, a further analogy with the intertropical shells that now live at the bottom of the ocean. ”

“ I select these, first, because they afford proofs of mechanical contrivances, more obviously adapted to a definite purpose, than can be found in shells of simpler character. Secondly, because the use of many of their parts can be explained, by reference to the economy and organization of the existing animals, most nearly allied to the extinct fossil genera and species with which we are concerned. And, thirdly, because many of these chambered shells can be shown, not merely to have performed the office of ordinary shells, as a defence for the body of their inhabitants ”