312 M. A. Milne-Edwards on the Carcinological undergo more than one change : tlius as these spicules are now composed of chalcedony and yet present the rhombohedral excavations of calcite on their surface, they must previously have been carbonate of lime ; and we know from their forms that they were originally siliceous spicules. Under what circumstances these alterations take place, or how they may occur, or why the mineral should be changed, must be a matter of conjecture ; but that they do occur we have evidence in the case just mentioned and in the formation of all mineral pseudomorphs ; so that, if the mould oi a Lithis-tid in flint, such as I have mentioned, were filled up with calcite and the flint subsequently removed, the original struc-ture, instead of being siliceous, would be calcareous, or it might be pyritic, and so on. In two parcels of powder which came from the interior of two separate Hints from Walling-ford, Berkshire, the Coccoliths, which abound in both, are all silicified in one, and all calcareous in the other. "Where the siliceous material of which the flints and chert are composed came from I do not pretend to say, any more than the calcareous material which formed the kunker, espe-cially the latter, seeing that out of eight analyses the quantity of lime only amounts to a mean of about nine parts in a thousand taken from the regur in eight different places, the lowest quantity of which, in three of the instances, did not reach two parts (Medlicott and Blanford, op. cit. vol. i. p. 430). It may, however, be fairly inferred that the purer material will be found in the nodular forms, both of flint and kunker, and the less pure in the tabular forms, viz. the sheet kunker and the chert respectively. Thus have I endeavoured to correlate that which may be said to be going on at the present day with what has taken place in ages past — not that such concretionary formations are confined to kunker and flint, for all geologists know that such have been taking place in the stratified deposits from the be-ginning ; but to comprehend all, so far as we are able, is best accomplished by studying what is taking place at the present moment for comparison with what has taken place heretofore, since this kind of induction is the least exposed to error. XXXI. — General Considerations upon the Carcinological Fauna of great Depths in the Carihhean Sea and Gulf of Mexico. By Alphonse Milne-Edwaeds*. The progress which submarine investigations have caused • Translated from the ' Coniptes Rendus,' February 21, 1881.
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