Transactions of the Geological Society, 1st series, vol. 2/On the Origin of Organic Impressions

XV. On the Origin of a remarkable class of Organic Impressions occurring in Nodules of Flint.

By. the Rev. William Conybeare, of Christ Church, Oxford, M.G.S.

THE suite of specimens accompanying the following notice are submitted to the inspection of the Geological Society, as illustrating the history of certain organic impressions which are found occurring in the flinty nodules of the chalk strata, and of which the real origin had previously escaped detection.

Mr. Parkinson, is I believe, the first naturalist who has noticed similar specimens; the following description of them is given in the second vol. of his valuable work on organic remains, and it is not possible to convey a more accurate idea of their external characters.

“Small round compressed bodies not exceeding the eighth of an inch in their longest diameters, and horizontally disposed, are connected by processes nearly of the fineness of a hair which pass from different parts of each of these bodies, and are attached to the surrounding ones; the whole of these bodies being thus held in connection.” Page 75, 76.

Mr. Parkinson proceeds to conjecture “that the formation of these bodies has been the work of some animal of a nature similar to the polypes by which the known Zoophytes are formed.” He therefore classes them among fossil corals of unknown genera, acknowledging at the same time that the circumstances which induced him thus to arrange them were very slight, and that they differed essentially from every zoophyte, recent or fossil, with which he was acquainted.

The impressions in question are generally found in a state of such indifferent preservation, and afford such insufficient data on which any conjectures concerning their formation can be founded, that we shall not be surprised to find the more perfect specimens now submitted to the Society. decidedly militating against the hypothesis proposed by the able fossilist above cited, and proving that the real origin of these bodies is widely different from that which he has assigned, they being in fact siliceous casts moulded in little hollow cells excavated in the substance of certain marine shells; the work perhaps of animalcules preying on those shells and on the vermes inhabiting them. It is almost unnecessary to add that these casts, like the screw stones of Derbyshire, must have been formed by the infiltration of the siliceous matter while yet in a fluid state into the cavities of the shells which it enveloped, and that they have been laid open and denuded in the specimens at present under consideration, by subsequent exposure to some agent capable of dissolving and removing the calcareous matter of the shell which formed the matrix, while the siliceous impression resisted it and remained unaltered.

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My first suspicion of these facts arose from a minute examination of the specimen represented in the drawings by fig. 1, Pl. 14. It presented several appearances which seemed to indicate the following conclusions.

  1. That the flat surface of flint over which the globules are in that specimen distributed, had been originally occupied by a large piece of the striated shell, the fragments of which occur so abundantly in the chalk-strata and accompanying flints, being very commonly considered as mutilated portions of a fossil pinna.
  1. That the globules themselves were casts moulded (in the manner already described) in cavities existing in the substance of the shell in question.

The most important of the appearances from whence these conclusions seemed to be deducible were the following.

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The flat surface occupied by the globular bodies at one extremity of fig. 2. is terminated by the perpendicular face or escarpment (if I may so employ that term) formed by a portion of the flint elevated a little above the rest; and this escarpment will be found marked with minute vertical striæ, seeming to indicate that it had been moulded against the edge of a fragment of the striated shell already alluded to; similar striæ are observable round the edges of another elevated portion towards the centre of the specimen, in some degree resembling the mill-marks round the edge of a coin, and they again occur on both sides of a small vein, or rather dyke of flint traversing a cluster of globules, and cutting many of them through the middle. This striated dyke had therefore all the characters of an impression formed in a long and narrow fissure of the shell in question, and was connected with the globular bodies in such a manner that the formation of the latter could scarcely be assigned to any other cause than that which produced the former.

In order to represent these appearances more clearly to the eye, several slight liberties have been taken in the drawing fig. 1. the relative position of some parts of the original specimen has been changed, and to the indications in question, a more prominent character than they really possess has been given. The small scale of the general outline fig. 1. not exceeding one of the original size, has rendered this more necessary; but in fig. 2. a detached representation of the siliceous dyke traversing the globular bodies has been added, on a scale rather larger than the original.

Desirous of pursuing the indications afforded by the circumstances above specified, I proceeded to examine many specimens of flint containing fragments of the striated shell in which the substance of the shell itself was still preserved, and in several instances observed evident traces of the siliceous globules of the cast, imbedded in their testaceous matrix; by submitting such specimens to the action of a diluted acid, that matrix was easily removed, and the casts themselves thus laid open by the destruction of their envelope, exhibited a perfect resemblance to the specimens previously described; the origin therefore which my former conjectures had induced me to attribute to those specimens now received the confirmation of a decisive experiment.

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The two compartments of fig. 3, exhibit representations of a specimen of the kind last described, before and after its denudation by the action of the acid. The specimen itself from which the latter draught was made, is submitted to the Society, together with the remainder of the mass of flint from which it was broken in an unaltered state.

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The succeeding figure (4.) exhibits, on a scale rather enlarged, a section carried vertically through a part of the specimen just described. The cellules forming the matrices of the globular casts will be seen in this section to have occupied nearly the whole thickness of the plate of shell. The numerous filaments extending between the globules in every direction, preserve the traces of as many minute tubes in which they must originally have been moulded, and it will be farther perceived that each globule is connected with the flinty envelope immediately above and below its centre by two thicker stems resembling the extremities of a projecting axis.

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The next specimen of the suite (fig. 5.) is a large fragment of some nondescript shell of the same striated structure; on a part of its fractured margin a range of hollow cellules will be observed similar to those which in the earlier specimens have been occupied by the infiltration of silex. In the present instance the specimen has been imbedded in chalk only, and the removal of that soft substance has exposed the cellules in their empty state.

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I have assigned the succeeding figure (6.) to a very interesting specimen, for the use of which I am indebted to Mr. Parkinson; it exhibits nearly the entire cast of a striated shell supposed by Mr. P. to be similar to that which Walch has described under the name of the Ostreo-Pinnite; the shell in which this cast was formed, must have become in places almost entirely cellular in its texture from numerous perforations of the kind so often alluded to, since the inferior margin of the upper valve is completely studded with a congeries of their globular casts of every size from that of the head of the smallest pin to discs of the eighth part of an inch in diameter.

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Fig. 7. represents a very delicate groupe of minute globules, which requires the assistance of a magnifying lens before its analogy with the preceding specimens can be perceived. To the naked eye it rather assumes the appearance of a coralloid; a magnified delineation of the same groupe is added, it is copied from a part of a large white flint found at Heytesbury, in Wiltshire, over the surface of which many such groupes are scattered, accompanied with impressions of fragments of the striated shell described by Da Costa as a patellite (Elements of Conchology, p. 142.): it is the flatter variety of the two which he describes, and it resembles that figured by Mr. Parkinson. Organ. Rem. Vol. 3. Pl. 5. fig. 3.

In all the preceding instances, the shells in which the cellules have been originally formed, have belonged to one class of fossil shells, the class (namely) which is distinguished by a striated texture similar to that of the recent pinna marina.

Owing to the great brittleness resulting from this texture, the specimens of such shells hitherto obtained have been found in too mutilated a state to authorise any definitive opinion with regard to their systematic classification. We may however securely venture to pronounce from the data in our possession, that there must certainly have existed more than one species, and most probably more than one genus of shells in which this structure prevailed; some of the fragments found may perhaps have belonged to a fossil pinna, others appear rather to resemble parts of a shell of the genus ostrea. Da Costa's conjecture formerly cited, which refers them to the patella does not seem to be grounded on sufficient evidence.

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Had the cellular excavations occurred only in shells of this class, concerning which our information is so imperfect, it might have been conjectured perhaps that they resulted from an original peculiarity in the organization of such shells, but the occurrence of an impression evidently moulded in similar cellules on the surface of a cast of the echinus has proved that this cannot be the case. The specimen alluded to is represented in fig. 9, the conjecture therefore originally proposed, namely that these cellules were the work of animalcules preying on the shells, and on the vermes inhabiting them seems to be the simplest manner of accounting for their formation.

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I might perhaps cite the specimen represented in fig. 8, for the same purpose with that last referred to, as affording another example of a similar cast formed in a body distinct from the striated shell; in this specimen a beautiful groupe of globules is seen occupying a conical hollow sunk into the substance of a flint pebble, and apparently formed by the impression of the pointed end of a belemnite. It is fair however to state, that together with fragments of the striated shell, certain testaceous bodies of a tapering cylindrical figure (which have seemingly constituted processes attached to such shells) are not infrequently found in the chalk strata, and that the last in question may possibly have been moulded in a fragment of this. The specimen itself is from the collection of Mr. Parkinson, to whose kind assistance during the course of these enquiries, I am happy to acknowledge myself much indebted.

I cannot conclude without apologizing to the Society for having occupied so much of its attention, by minute details which I fear may have appeared extremely jejune and uninteresting, as they are confined to the illustration of an insulated fact in the history of organic remains, in itself of very inferior importance, and cannot be said to throw any additional light on the general views even of that branch of geological science with which they are connected.

Since writing the above I have received from my friend Mr. Buckland the following observations relative to this subject, accompanied by a recent specimen which appears to illustrate them in a very satisfactory manner.

“The hollows that afforded a mould for the formation of these singular bodies, appear to me to have been the work of some minute parasitical insect. The small aperture, the cast of which now forms the projecting axis of each globule, was probably perforated by this intruder as the entrance to his future habitation; having completed this passage, and excavated at its termination a cell suited to his shape and convenience, he appears by the aid of a delicate auger or proboscis to have drilled many minute and almost capillary perforations into the substance of the shell on every side around him, taking care to leave always partitions sufficient to support the thin external plate of shell which formed the roof of his apartment. Having exhausted all the nourishment which could in this manner be procured with safety from the vicinity of this first establishment, the insect appears to have emigrated, and after working for itself a lateral passage to a sufficient distance, to have formed a new settlement in the midst of fresh supplies. In the recent oyster shell which I have transmitted, you will perceive that this process has been carried on to a great extent, in the intermedial matter between two or three sets of the pearly plates comprising it; and yet without effecting the destruction of the exterior crust, or in any degree injuring the inner surface of the shell, which remains untouched, and, notwithstanding these attacks, still equally adapted to every purpose required by the œconomy of its inhabitant.