Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/339

Rh 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.

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.

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