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. 



