The grounds upon which this supposition is made, is the observation that the recent analogues of most of these fossils are now inhabitants of the torrid zone. The extreme delicacy however of most of these shells, as well as their perfect preservation (as Mr. Parkinson has already observed,) precludes entirely the possibility of their having been brought from distant places, and they serve merely to shew that this part of the globe must be very different, relatively to such species of animals, from what it was at the period of their entombment.
In some beds of pebbles and gravel fossil shells are very numerous, as in those of Woolwich, Harwich, &c. and with these alternate frequently beds of clay and sand, containing shells regularly distributed, entire, and apparently undisturbed. Of these shells Mr. Parkinson observes, some belong to species now only found in distant seas, and others appear “not to differ specifically, as far as their altered state will allow of determining, from the recent shells of the neighbouring sea.”
But granting, as we must, that the formation of pebbles has taken place at different periods, it must be extremely difficult, and perhaps impossible in all cases, to distinguish whether they are now to be seen in the places where they were at first deposited, or whether they may not have repeatedly been moved.
In many beds of gravel delicate shells are so abundant that the cause which placed them there could not have been very violent; whilst others are found totally destitute of organic remains, except such as are impressed upon the substance of the pebbles themselves. Among the latter may be enumerated, pectines, anomiæ, the interior casts of echini, and impressions of the spines and plates; zoophytes of unknown genera, some resembling alcyonia, are also frequent. It does not appear certain whether all these may not have
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